Oauth school selector: Allow to chose 'no school' in school selector in order to allow to retreive old data

Admin interface: Set correct csrf tokens so groups and participations can now be deleted. Fixes #116

Admin interface: Make name search case insensitive in postgres, make oauth id search also look for oauth id without school id

UX tweaks: Template jwinf: Enhance notes for logincodes, admin interface: Show less id and more information about student / teacher / admin status

Oauth login: Throw error instead of silently ignoring school information not being configurerd on teacher login via PMS