Use standard infrastructure for Audit email generation

Summary: Ref T4896. Replace custom stuff with standard stuff.

Test Plan:
  - Sent a bunch of email and it all looked sensible/correct.
  - Made sure to test inlines, specifically, as they're a bit tricky.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10112