Normalize the definition of "closed" revision statuses

Summary:
Currently, "Closed" and "Abandoned" are treated as "closed". I want to add a flag which treats "Accepted" as "Closed", too, for Asana and other companies who use an Asana-like workflow.

The background here is that their workflow is a bit weird. They basically do audits, but have a lot of things which Diffusion doesn't do well right now. This one change makes Differential fit their workflow fairly well, even though it's an audit workflow.

To prepare for this, normalize the definition of "closed" better. We have a few callsites which explicitly check for "ABANDONED || CLOSED", and normalizing this is cleaner anyway.

Also delete the very old COMMITTED status, which has been obsolete for over a year.

Test Plan: Browsed around most/all of the affected interfaces.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7653