Parse ICS datetimes and durations

Summary:
Ref T10747. This parses "SUMMARY" and "DESCRIPTION" from ICS (easy) and "DTSTART", "DTEND" and "DURATION" (big mess, as one might reasonably expect).

ICS dates can be absolute (e.g., at a specific time in history) or "floating". If an event is "floating", it has a date like "June 23 at 5PM", which is expected to be interpreted according to the viewer timezone, i.e. occur at different times in different timezones.

Dates can also be "start + end" or "start + duration". Worse, they can be a floating start plus a duration. Do a bunch of gymnastics to represent all this stuff faithfully.

This probably still doesn't handle all-day events properly, but I'll deal with them in the future.

Test Plan: Added unit tests; ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16548