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 phorge/src/applications/calendar/editor/PhabricatorCalendarImportEditEngine.php on line 45
Before this change it was only theorically possible to edit the name, policies, etc.
but not the URI.
This change also introduces the ability to edit the specific ICS URI,
in order to change legitimate parts of the URI, like the secret token.