Have TransactionComments return a PHUIObjectBoxView

Summary: Simplifies the code a bit and fixes all the wonky previews. Fixes T4053

Test Plan: Test all pages, logged in and logged out.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4053

Differential Revision: https://secure.phabricator.com/D7622