R1:41f0b8b0a3a1

Allow subtypes to specify "mutations", to control the behavior of the "Change Subtype" action

Summary:
Fixes T13415. Provide a way for subtypes to customize the behavior of "Change Subtype" actions that appear above comment areas.

Subtypes may disable this action by specifying `"mutations": []`, or provide a list of subtypes.

The bulk editor and API can still perform any change.

Test Plan:
- Tried to define an invalid "mutations" list with a bad subtype, got a sensible error.
- Specified a limited…