PhabricatorMemeEngine HA HA HA HA

Summary:
Depends on D19198. Ref T13101. Ref T5258. Pull compositing logic out of the Controller.

This is moving toward fixing memes in email.

Test Plan: Used new and old memes. Used API memes.

Maniphest Tasks: T13101, T5258

Differential Revision: https://secure.phabricator.com/D19200