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.

• 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).