A closed commit can be reopened, if allowed by the config file.

Summary: Fixes T2316

Test Plan:
When the config file allows reopening,
navigate to a closed revision and reopen it in the user interface,
and verify that the revision now "needs review."
Also checks that the reopen option is unavailable when disallowed
by the config file.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2316

Differential Revision: https://secure.phabricator.com/D4526