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.
{F1830577}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10747
Differential Revision: https://secure.phabricator.com/D16553