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