Add a setting to blanket-disable autoclosing revisions in a repository

Summary: For git workflows where developers push personal feature branches to the origin, this is a quick workaround until T1210 is implemented properly. I'll also mention this in D2446.

Test Plan:

• Committed a revision in an "autoclose" repository; it was autoclosed. Committed a revision in a no-autoclose repository, it was not autoclosed.
• Edited repositories, saving the "autoclose" setting as enabled/disabled.

Reviewers: aurelijus, btrahan

Reviewed By: aurelijus

CC: aran

Maniphest Tasks: T1210

Differential Revision: https://secure.phabricator.com/D2448