Make Maniphest task statuses user configurable

Summary: Fixes T1812. Moves the internal configuration into public space and documents it.

Test Plan:
  - Tried to set it to some invalid stuff.
  - Set it to various valid things.
  - Browsed around, changed statuses, filtered statuses, viewed statuses, merged duplictes, examined transaction record, created tasks.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1812

Differential Revision: https://secure.phabricator.com/D8585