implement roles

Users will be assigned to roles. What needs to be defined on a WIKI page is a list of roles we would like to have (maybe derived from the roles in Wordpress). @jonas.liebst

We then can implement those roles in Keycloak and try to map the roles to the clients (WP, CBA, PeerTube).

For Wordpress, roles can be mapped here https://displeu.dev.osalliance.com/wp-admin/options-general.php?page=openid-connect-generic-settings

For PeerTube, a login role can be mapped in the OIDC plugin settings like here https://fair.tube/admin/plugins/show/peertube-plugin-auth-openid-connect (to be tested)