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