render_tag -> tag: Differential Changeset stuff

Summary: I cheated in a couple of places here, but this is in the process of getting refactored anyway, and there's a pretty clear boundary.

Test Plan: Viewed changesets in Differential, viewed standalone. Viewed context elements.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4723