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 + !unsubscribe to pastes.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12236