R1:6984d239b02b

Convert Maniphest custom config to new config types

Summary:
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.

Test Plan: Edited, deleted and mangled all the…