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 mutations list and an empty mutations list, verified that corresponding tasks got corresponding actions.
• Used the bulk editor to perform a freeform mutation.
• Verified that tasks of a subtype with no "mutations" still work the same way they used to (allow mutation into any subtype).

Maniphest Tasks: T13415

Differential Revision: https://secure.phabricator.com/D20810