R1:d8bbf559592f

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…