Fixes failing start up because of no longer existent
`security.guest.lecturer.enabled` setting.

Extend API and config to support auth services per role



See merge request !22

evict session cache both for session entries in the cache (https://github.com/th…

…m-projects/arsnova-backend/wiki/Caching  <-- see List of cache entries and associated keys)

See merge request !20

(https://github.com/thm-projects/arsnova-backend/wiki/Caching
^-- see List of cache entries and associated keys)

add bulk save method for questions (for importing via csv)

This merge request adds a bulk save function for questions. For now, it's only used for the question import via csv to not save every single question with a seperate request.

See merge request !19