Convert Calendar Events to use RRULE frequency constants in various other places

Summary:
Ref T10747.

• Store recurrence as RRULEs internally.
• Use RRULE constants.
• Migrate existing rules to RRULEs.

Test Plan: Ran migration, nothing seemed broken?

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16670