Fix RRULE bugs with week numbers for leading and trailing partial weeks

Summary:
Ref T10747. I only got this partially right:

• When the last week of the year has less than 4 days, it's counted as the first week of the next year.
• When the first week of the year has less than 4 days, it's counted as the last week of the prior year.

These rules are sort of crazy, but they work now.

Test Plan: Unit tests now pass.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16611