Streamline tag rendering for Differential and Maniphest

Summary:
Well, I'm just putting it into the DAO classes, or am I doing something wrong?

Will be used by future event listeners

Test Plan: Visited some tasks and revisions. Look fine.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5542