Restructures the code base with more object methods

[kushaldas]

Moves configuration related code and also callbacks as proper methods into Proxy class.

proxy.py

Now, Proxy class gets a must conf_path argument, and it creates the inital
`conf` attribute from that.  Proxy also has default on_save and on_done and
err_on_done methods.  def handle_response(self) method has a `assert
self.res` to mark that res is populated before this call. This is for mypy

main.py

Mostly has changes from black

entrypoint.py

No need for fancy dynamic err_call_back, the proxy object will call
self.err_call_back if any issue in reading configuration.

The test cases now have their own configuration files to create the proxy
object. Also, to do proper dynamic attachment of any method of Proxy class
we are using https://docs.python.org/3/library/types.html#types.MethodType
so that our own on_save or on_done or err_on_done will be called during
tests.

stretch test will fail as it does not have any variable level type annotation.

Testing

1. Build the securedrop-proxy source distribution:
   • check out this branch
   • run python setup.py sdist
2. Build the .deb:
   • in a clone of securedrop-debian-packaging, run PKG_PATH=/home/user/src/securedrop-proxy/dist/securedrop-proxy-0.1.6.tar.gz PKG_VERSION=0.1.6 make securedrop-proxy (adjust the path to your working copy of this repo as necessary)
3. Install the .deb in sd-proxy-buster-template:
   • qvm-copy ~/debbuild/packaging/securedrop-proxy_0.1.6+buster_all.deb and specify sd-proxy-buster-template as the target VM
   • In a shell on sd-proxy-buster-template, run:
     • cp ~/QubesIncoming/sd-dev/securedrop-proxy_0.1.6+buster_all.deb /tmp
     • sudo apt install /tmp/securedrop-proxy_0.1.6+buster_all.deb
4. Reboot.
5. Start the SecureDrop client in sd-svs and confirm that the proxy is communicating with the server: submissions and replies should populate with no errors.

[redshiftzero]

note that we should merge freedomofpress/securedrop-builder#120 first otherwise we'll break stretch nightlies

[redshiftzero]

this looks great! diff lgtm here but I noticed that the test plan involved installing the deb to sd-svs whereas we want it in sd-proxy, I've updated it - can you confirm you tested on sd-proxy? whichever of you @rmol or @kushaldas confirms that just note in a comment on this PR

[kushaldas]

I copy pasted in sd-proxy and tested.

[redshiftzero]

diff lgtm, and gave this in a spin in Qubes, all good - thanks for these changes!

@rmol I'll wait before merging so you have a chance to comment on this

[rmol]

Looks good, except for one preexisting problem your changes made me notice, and a check that's obsoleted by your work. I did not test in Qubes since @redshiftzero already did.