Add a very forgiving GC for Differential viewstate information

Summary:
Ref T13455. Viewstates are fairly small and will probably grow less quickly than the changeset table, but the data is also not important to retain in the long term: if you revisit a change several months after hiding some files, it's fine if we've forgotten that you adjusted the view parameters.

Add a GC with a long default collection policy (180 days) so installs can manage the size of this table if it becomes necessary.

Test Plan: Ran via bin/garbage to adjust the GC policy and collect viewstates.

Maniphest Tasks: T13455

Differential Revision: https://secure.phabricator.com/D21164