Add inline comments to diffusion/audit emails

Summary:
Depends on D1929. In emails, notify recipients that inlines are attached.

Vaguely copy/pastey from Differential but they only share like six lines and this seems like a random piece of code to pull out.

Test Plan: Added inline comments, got email mentioning them

Reviewers: davidreuss, nh, btrahan

Reviewed By: davidreuss

CC: aran, epriestley

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D1930