Track closed date and closing user for tasks explicitly

Summary:
Ref T4434. Although some of the use cases for this data are better fits for Facts, this data is reasonable to track separately.

I have an approximate view of it already ("closed, ordered by date modified") that's useful to review things that were fixed recently. This lets us make that view more effective.

This just adds (and populates) the storage. Followups will add Conduit, Export, Search, and UI support.

This is slightly tricky because merges work oddly (see T13020).

Test Plan:

• Ran migration, checked database for sensible results.
• Created a task in open/closed status, got the right database values.
• Modified a task to close/open it, got the right values.
• Merged an open task, got updates.

Maniphest Tasks: T4434

Differential Revision: https://secure.phabricator.com/D19037