Add an "Export as .ics" action to Calendar events

Summary: Ref T10747. Allows you to grab an event as a (basic) ICS file.

Test Plan:

• Exported a normal event.
• Exported an all-day event.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16553