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 + `!unsubscribe` to pastes.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T7199
Differential Revision: https://secure.phabricator.com/D12236
