Add object status to Handles

Summary:
We use ObjectHandles as proxy objects which can refer to any other object in the
system. Add the concept of the underlying object's "status" (e.g., open, closed
or busy).

This allows us to render completed tasks and revisions with strikethrough. In
the future, if we implement OOO or something, we could render users with a
"busy" status if they're on vacation, etc.

Test Plan: Viewed a task with closed revisions and dependencies:
https://secure.phabricator.com/file/view/PHID-FILE-6183e81286fa3288d33d/
Reviewed By: codeblock
Reviewers: codeblock, hunterbridges, jungejason, tuomaspelkonen, aran
CC: aran, codeblock
Differential Revision: 772