The testing section is more appropriate, since it's fundamentally part
of our CI system.
While we're at it, fix the fact that we were linking to GitHub, not
ReadTheDocs, in the run-mypy output.
A bunch of details were out of date about the current state of Zulip
and thus what projects make sense; while this isn't likely to be our
final 2019 ideas list, this should help avoid confusion about what's
happening.