Improve behavior when user submits a no-op action in Differential

Summary:
See T730 and the slightly-less-pretty version of this in D1398.

When a user takes an action in Differential that has no effect (for instance,
accepting an already-accepted revision), prompt them:

Action Has No Effect

You can not accept this revision because it has already been accepted.

Do you want to post the feedback anyway, as a normal comment?

                      [Cancel] [Post as Comment]

If they have no comment text, the dialog only says "Cancel".

I think this is probably the best way to balance all the concerns here -- it
might occasionally be a little annoying, but that should be rare, and it should
never be confusing (the current workflow is extremely confusing).

This also fixes the issue where you can add all sorts of CCs who are already
part of the revision, either explicitly or via mentions.

Test Plan:
Posted some has-effect and has-no-effect comments, made different
choices in the dialog, everything seems to work OK?

Reviewers: vrana, btrahan, jungejason

Reviewed By: vrana

CC: aran, vrana

Maniphest Tasks: T730

Differential Revision: https://secure.phabricator.com/D1403