Integration test for yarrharr executable, release process

[twm]

The v0.7.0 release was DoA due to a bytes/str mismatch in the twisted.web routes (specifically the code did self.putChild('static') instead of self.putChild(b'static')). Add an integration test that verifies that the release artifacts function properly. At the least:

1. Do a "make release" to generate a distribution (to verify that works).
2. Install the distribution into a virtualenv (to verify that works).
3. Start the yarrharr executable (to verify it starts).
4. Do a GET / and verify that there are no dead links.

Some day maybe WebDriver or something, but at the moment the UI is changing too quickly for that.

[twm]

A short smoke test would also be useful:

1. Login (would have caught Set CSRF_TRUSTED_ORIGINS #932 breaking login)
2. Add a feed
3. Wait for the feed to fetch
4. View the feed content