R1:1965b78b34f1

Fix subtype extension support check

Summary: Fixes T13663. `supportsSubtypes` tries to create an editable object, but this isn't always valid for `PhabricatorCalendarImport`. Use `instanceof` instead.

Test Plan:
- Edited calendar import, tasks (2 different subtypes), and projects (2 different subtypes).
- Changed task subtypes using {nav Change Subtype} action and batch editor.
- Changed task and project subtypes using Conduit.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley…