Use ModularTransactions for accept/reject/resign in "differential.createcomment"

Summary:
Ref T10967. differential.createcomment is a frozen API method which has been obsoleted by differential.revision.edit.

It is the only remaining way to apply an "accept", "reject", or "resign" action using the old "ACTION" code.

Instead of using the old code, sneakly apply a new type of transaction in these cases instead.

Then, remove all the remaining old code for this stuff on the write pathways.

Test Plan:

• Used "differential.createcomment" to accept, reject, and resign from a revision.
• Grepped for all removed ACTION_X constants, found them only in rendering code.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10967

Differential Revision: https://secure.phabricator.com/D17513