Render Differential previews through ApplicationTransactions

Summary: Ref T2222. Use new code for rendering. Delete `DifferentialRevisionCommentView`, which has no remaining callsites.

Test Plan: Went through all the different actions and verified the previews rendered correctly. Reloaded page to test draft behavior.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8236