Parse (and discard) VALARM sections in ICS files
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.
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D16761