On inline comments, track an explicit "committed" content state

Summary:
Ref T13559. To allow the client to make correct decisions about what buttons mean, track an explicit "Committed" content state.

This is the last version of the comment that has been saved on the server, and does not exist if the comment has never been saved.

Test Plan: Created comments, etc. See followups.

Maniphest Tasks: T13559

Differential Revision: https://secure.phabricator.com/D21651