Support RRULE "COUNT" for recurring events

Summary:
Ref T10747. RRULE events can repeat "UNTIL" a certain time, or a certain "COUNT" of times.

In the UI, we only support "UNTIL". Also support "COUNT".

Test Plan: Imported an event which repeats every other day, 5 times. Got 5 instances.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16749