Make Pholio mail render without a ton of over-escaped HTML

Summary:
Ref T13202. See PHI900. Fixes T12814. Pholio currently builds HTML comments in an older way that can dump a bunch of over-escaped HTML into mail bodies.

Update the logic to be more similar to the Differential rendering logic and stop over-escaping things.

The result isn't perfect, but is dramatically less broken.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13202, T12814

Differential Revision: https://secure.phabricator.com/D19733