Add a config flag to treat "Accepted" revisions as "Closed"

Summary: See D7653. This is exclusively for Asana, who uses Differential for a post-commit, Audit-like workflow but has a small set of requirements for it to be a good fit (just this) and a large set of requirements for Diffusion/Audit to be a good fit.

Test Plan: Set the flag, verified "Accepted" revisions are no longer on the dashboard.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7654