Add initial support for complex recurring events

Summary:
Ref T10747. In .ics files, event recurrence can be defined in three ways:

• RDATE, a list of specific dates when the event occurs.
• EXDATE, a list of specific dates when the event does not occur. This is the opposite of RDATE.
• RRULE, a complex description of recurrence rules, like "Every other Monday in April and March".

We currently use a simpler recurrence approach internally, but I plan to implement (nearly) full-power RRULE-like recurrence and use that as the internal recurrence engine, then just expose a simpler UI (this is similar to what other applications do).

To start with, define everything except the RRULE stuff, since that's much harder:

• A Source is something that can generate recurring dates.
  • A List is a source with specific dates (RDATE or EXDATE).
  • A Rule is a source which implmenets RRULE behavior. This change has a skeleton, but no code.
• A Set is a collection of sources (e.g., zero or more sources, often zero or one RDATE, zero or one EXDATE, and zero or one RRULE).

Basically, this gets all the support code in place to actually write RRULE stuff, but doesn't actually implement it yet.

Test Plan: Unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16590