Transactions - make sure to do fancy remarkup stuff on edit too

Summary: Fixes T6648. We do some automagical hotness based on the text you enter in remarkup textareas - e.g. adding projects or mentioning other objects. Refine the code here so that even when just editing a comment we build these transactions and apply them.

Test Plan: edited a comment and noted new mentions and projects showed up appropriately...!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6648

Differential Revision: https://secure.phabricator.com/D10922