Keycloak as default IDP for translate5

Keycloak should be integrated as default IDP in translate5's docker setup.

Users should still be maintained in translate5, but in the default setup synced with Keycloak.

Reason: For chat integration we need an IDP for user authentication.

If another IDP is used together with translate5 via existing OpenID Connect integration, users of that other IDP are synced to translate5 (current status) and then synced from translate5 to Keycloak.

Also for those setups Keycloak must be used, because otherwise users created in translate5 itself will not be able to use the chats.

With integrating default SSO IDP for translate5 we should also implement tests.