Add "ORGANIZER" and "ATTENDEE" sections to ICS writer

Summary: Ref T10747. Allows us to export organizer/host and invitee/attendee information in .ics files.

Test Plan:
Added unit tests, ran unit tests.

{F1832200}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16557