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 Change Subtype action and batch editor.
• Changed task and project subtypes using Conduit.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Maniphest Tasks: T13663

Differential Revision: https://secure.phabricator.com/D21714