R1:96f800249b31

Export ORGANIZER and ATTENDEE sections in ".ics" files from Calendar

Summary: Ref T10747. This exports these sections when generating an ".ics" file.

Test Plan: {F1832214}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16558