[AP_05] OpenID Connect authentication

[AP_05] Als <user> möchte ich mich mit den THM_Credentials anmelden können

After our meeting with Michael Fey from ITS earlier today, @mlbn33, @dmlk31 and I agreed on using OpenID Connect instead of the current OAuth2 authentication (#114 (closed)). This approach has the added benefit of making #102 (closed) more or less obsolete because we will no longer need to rely on client secrets in the app.