Convert Maniphest custom config to new config types
Technically, this addresses the core issue in T12845 too, but I'm going to convert the rest of the custom:... types before closing that.
In particular, for T12870:
- Validates that keywords are unique across priorities.
- Fixes missing newline in documentation.
- Updates documentation to note that keywords are now mandatory and must be unique across priorities.
Test Plan: Edited, deleted and mangled all the Maniphest custom options (priorities, statuses, points, subtypes).
Reviewers: chad, amckinley
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D18165