Make the client authoritative for "Cancel" actions

Summary:
Ref T13559. When the user clicks the "Cancel" button, we sometimes take it to mean "delete" (when the comment is empty).

Both the client and server make a decision about this, and they may not agree, which causes the client to fall out of sync.

Make the client responsible for deciding whether it wants to interpret a click on the "Cancel" button as a "revert" or a "delete".

Test Plan: Cancelled empty and nonempty comments, etc. See followup changes.

Maniphest Tasks: T13559

Differential Revision: https://secure.phabricator.com/D21649