Protection for participant managers, Issue #122

Raises a validation error, if a non-superuser user edits himself and
removes the last group containing the permission to manage participants
Merge pull request #591 from normanjaeckel/WarningRechtAufRechte

Protection for participant managers, Issue #122
New Overlay API
Merge pull request #592 from ostcar/overlay

New Overlay API
fixed error on new overlay api
Merge pull request #593 from ostcar/master

Fixed Error in new Overlay API