R1:02a4f8b0c8f1

Fix editing a Calendar import ICS URI

Summary:
Before this change it was only possible to create a Calendar import ICS URI.
After this change it's possible to also edit already-existing elements.

This change fixes this specific exception when visiting similar pages:

/calendar/import/edit/5/

Argument 2 passed to PhabricatorCalendarImport::initializeNewCalendarImport()
must be an instance of PhabricatorCalendarImportEngine, null given, called in…