Fixes T12870. Ref T12845.

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.

Edited, deleted and mangled all the Maniphest custom options (priorities, statuses, points, subtypes).