Make the client authoritative for "Cancel" actions
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