Web API: Small tweaks from PR 301

[tadhg-ohiggins]

This issue is for the comments CM made in #301 that aren't big enough to warrant having individual issues for them.

• Make what build_eregs_args returns clearer in its docstring.
• Make if/else early-exit branching cleaner/clearer in FileUploadView.
• Use six.moves.urllib.parse.urlparse instead of the try/except approach I'm using at the moment.
• Use Django safe string methods (or six equivalents?) for handling hashed file contents instead of the try/except I'm currently using.
• Add port 443 as a "hidden" port in the same way 80 is in utils.get_host.
• Switch _fake_redis_job to returning namedtuple("FakeRedisJob", ['id'])(fake_pipeline_id) instead of its current type?

I addressed the above in #308.

• Look at use of sub_path as argument to status_url, and the fact that the code currently has to “tell” view controllers what their path is, which seems odd.
• Fix trailing (/) in URL regular expressions.
• Make sure the current code isn't changing querysets into plain lists, and/or address problems arising from that if it is doing so.
• Use the tmpdir fixtures from py.test—current code leaves tempfiles all over the repo root directory. (Note that these are likely the uploaded Django temporary files, but still, a better answer to this would be good.)