alembic migrations are broken because alembic doesn't support drop_column for sqlite

[heartsucker]

To reproduce:

rm -rf svs.sqlite
sqlite3 svs.sqlaite .databases
alembic upgrade +2
alembic downgrade -1

Output from the last command:

INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
INFO  [alembic.runtime.migration] Will assume non-transactional DDL.
INFO  [alembic.runtime.migration] Running downgrade cd0fbb73bf8e -> 5344fc3a3d3f, Capture document count
Traceback (most recent call last):
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1193, in _execute_context
    context)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/engine/default.py", line 509, in do_execute
    cursor.execute(statement, parameters)
sqlite3.OperationalError: near "DROP": syntax error

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/bin/alembic", line 11, in <module>
    sys.exit(main())
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/config.py", line 502, in main
    CommandLine(prog=prog).main(argv=argv)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/config.py", line 496, in main
    self.run_cmd(cfg, options)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/config.py", line 479, in run_cmd
    **dict((k, getattr(options, k, None)) for k in kwarg)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/command.py", line 294, in downgrade
    script.run_env()
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/script/base.py", line 427, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/util/pyfiles.py", line 81, in load_python_file
    module = load_module_py(module_id, path)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/util/compat.py", line 82, in load_module_py
    spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 673, in exec_module
  File "<frozen importlib._bootstrap>", line 222, in _call_with_frames_removed
  File "alembic/env.py", line 70, in <module>
    run_migrations_online()
  File "alembic/env.py", line 64, in run_migrations_online
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/runtime/environment.py", line 836, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/runtime/migration.py", line 330, in run_migrations
    step.migration_fn(**kw)
  File "/home/heartsucker/code/freedomofpress/securedrop-client/alembic/versions/cd0fbb73bf8e_capture_document_count.py", line 25, in downgrade
    op.drop_column('sources', 'document_count')
  File "<string>", line 8, in drop_column
  File "<string>", line 3, in drop_column
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/operations/ops.py", line 1667, in drop_column
    return operations.invoke(op)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/operations/base.py", line 319, in invoke
    return fn(self, operation)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/operations/toimpl.py", line 81, in drop_column
    **operation.kw
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/ddl/impl.py", line 172, in drop_column
    self._exec(base.DropColumn(table_name, column, schema=schema))
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/alembic/ddl/impl.py", line 115, in _exec
    return conn.execute(construct, *multiparams, **params)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 948, in execute
    return meth(self, multiparams, params)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/sql/ddl.py", line 68, in _execute_on_connection
    return connection._execute_ddl(self, multiparams, params)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1009, in _execute_ddl
    compiled
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1200, in _execute_context
    context)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1413, in _handle_dbapi_exception
    exc_info
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/util/compat.py", line 265, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/util/compat.py", line 248, in reraise
    raise value.with_traceback(tb)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 1193, in _execute_context
    context)
  File "/home/heartsucker/.local/share/virtualenvs/securedrop-client-GExBtLKv/lib/python3.5/site-packages/sqlalchemy/engine/default.py", line 509, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) near "DROP": syntax error [SQL: 'ALTER TABLE sources DROP COLUMN document_count'] (Background on this error at: http://sqlalche.me/e/e3q8)

Note that this would have been caught by #55

We can fix this by copying what we do in SecureDrop core like we do here with temp tables: https://github.com/freedomofpress/securedrop/blob/develop/securedrop/alembic/versions/2d0ce3ee5bdc_added_passphrase_hash_column_to_.py