Adds Remarkup Rendering to Calendar Events' Descriptions.
Summary: Closes T8032
Test Plan: Verify that when editing a calendar event's description, there is a UI bar helping with adding markup to the description. Also verify that markup is displayed correctly on the event page once the event has been updated.
Reviewers: lpriestley, Blessed Reviewers, epriestley
Reviewed By: lpriestley, Blessed Reviewers, epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T8032
Differential Revision: https://secure.phabricator.com/D13355