Process Remarkup in text and HTML email bodies appropriately

Summary: Ref T6343, adding HTMLMailMode to remarkup, and most objects should now be processed and appear pretty in emails.

Test Plan: Add a comment to a Maniphest task containing a mention of an object like '{T1}' or 'T1'. Emails should show a styled version of the object similar to how the object looks in the context of the Maniphest task in the UI.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: chad, Korvin, epriestley

Maniphest Tasks: T6343, T2617

Differential Revision: https://secure.phabricator.com/D10859