Convert database user IDs to lowercase on account creation an login.
Additionally, LDAP UIDs are lowercased as a fallback when they cannot be
retrieved from the server.

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)