Fix incorrect construction of subtype map when validating "subtype" transactions against non-subtypable objects
Fixes T13389. Currently, we try to "newSubtypeMap()" unconditionally, even if the underlying object does not support subtypes.
- Only try to build a subtype map if subtype transactions are actually being applied.
- When subtype transactions are applied to a non-subtypable object, fail more explicitly.
Test Plan: Clicked "Make Editable" in a fresh Calendar transaction form, got an editable form instead of a fatal from "newSubtypeMap()". (Calendar events are not currently subtypable.)
Maniphest Tasks: T13389
Differential Revision: https://secure.phabricator.com/D20741