Remove timeouts for API calls from the client and use SDK for timeouts

[heartsucker]

Fixes #323. Fixes #324. Fixes #325. Toward #326. Fixes #327.

This is the first pass that shows that we can easily remove the timeouts from the client. This is only toward #326 and doesn't fix it since we should add logic to extend the timeout to be even longer for very large files.

Blocked by freedomofpress/securedrop-sdk#80 (PR that adds timeouts to the SDK itself).

Testing

To test this, login. Then, go into SD core, go into journalist_app/api.py, and in setup_g add import time; time.sleep(30). Save the file. Flask will refresh. Now all requests will be delayed by 30 seconds, and the SDK timeout is 20. This will cause all requests to raise a timeout error. Check the logs for the presence of this error.

[redshiftzero]

I haven't tested this, just reviewed the diff, but this approach seems much cleaner and simpler to me (and to the interested observer this will also resolve #325 and #323) 🙌

[heartsucker]

Because of the longer timeouts in the SDK for downloads, this also gets #326. We also get #324 because we aren't using QThreads at all.

[sssoleileraaa]

💥 Lotta "Fixes" in this PR 💥

Note: to test this, I updated setup_g in journalist_app/__init__.py to cause a timeout and saw the expected error message in the error status bar in the client.

LGTM