In Calendar, only include the event description in the original event mail

Summary: Ref T11809. This makes the mail more consistent with Differential and Maniphest, which only include additional details in the first mail in the thread.

Test Plan:

• Created an event with a description.
• First mail included it.
• Followups did not.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11809

Differential Revision: https://secure.phabricator.com/D16794