HTML emails for Calendar event description changes should respect remarkup rules

Summary: Ref T7964, HTML emails for Calendar event description changes should respect remarkup rules

Test Plan: Create event and edit description, check that email has a correctly formatted remarkup description section.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T7964

Differential Revision: https://secure.phabricator.com/D13554