Fix incorrect construction of subtype map when validating "subtype" transactions against non-subtypable objects

Summary:
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