Bitbucket has sadly removed their Mercurial support, so we're doing the
same with our Bitbucket support.

Older repositoriy configurations in Review Board will still remain, but
of course are no longer accessible. We disallow configuring new
Mercurial-based repositories, though, leaving in only Git.

Testing Done:
Unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/11041/