Maniphest + Differential - don't show transaction preview until user interacts with the form.

Summary: Fixes T4846. These are one off (for now) since they have various crazy actions with them. I think this will get unified and more cleaned up when we refine the UI for taking multiple actions at once, etc.

Test Plan: noted no "commented on x" in either maniphest or differential. starting making a comment and noted prevew showed. started adding a subscriber (added to tokenizer) and preview showed.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4846

Differential Revision: https://secure.phabricator.com/D12936