[Registrant Element] Automatically open create subform

[dpi]

If there is no 'existing' permission, but there is 'creation' permission, then automatically 'open' the create subform instead of requiring button press.

(only do this if there are no registrants yet / or using single, otherwise it kills entire form validation )

[JPustkuchen]

+1 for this. It may be good in combination with #158