Parse (and discard) VALARM sections in ICS files

Summary:
Ref T10747. Ref T11790. These sections may legally appear inside VEVENT sections.

For now, parse but discard them. At least for now, we assume whatever you're importing from is authoritative and handles notifying you if you're importing events.

(It also probably makes more sense to set per-import or per-user notification settings than trust VALARMS? But we can cross that bridge when we come to it.)

Test Plan: Added unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747, T11790

Differential Revision: https://secure.phabricator.com/D16761