Allow Maniphest statuses and priorities to be disabled

Summary: Fixes T9496. If you have some statuses or priorities you don't need, allow users to disable them to stop the bleeding.

Test Plan:

• Set task to status X and priority Y.
• Disabled X and Y using config.
• Verified task still had old status/priority.
• Verified new task could not be created/edited into those settings.
• Verified task/priority appeared in typeahead, but were marked as disabled.
• Viewed email command docs.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9496

Differential Revision: https://secure.phabricator.com/D14681