Add very basic tag support to Diffusion
Summary: Lists the 25 most recent tags on the "Repository" page.
Test Plan: Looked at a git repository with a tag, saw it. Looked at HG/SVN repos, they didn't break.
Reviewers: davidreuss, 20after4, btrahan, vrana, jungejason
Reviewed By: davidreuss
CC: aran
Maniphest Tasks: T1130
Differential Revision: https://secure.phabricator.com/D2255