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.
- 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
Maniphest Tasks: T13663
Differential Revision: https://secure.phabricator.com/D21714