R1:161f936871b8

Lift common code for transaction-based reply handlers into parent class

Summary:
Ref T7199. Essentially all of the reply handlers now apply transactions to something which implements PhabricatorApplicationTransactionInterface.

We can share code between them by lifting this stuff into a superclass.

First, convert paste. Also rename `PasteMockMailReceiver` to `PasteMailReceiver` (this got mis-copied from Pholio at some point, I think).

Test Plan: Used `bin/mail receive-test` to send comments + `!