Instead of only syncing groups that have an external id set, we instead match
the names of groups that we get via OAuth2 from Keycloak against the names of
all local groups.

If the user data provided contains a group that exists locally, the user is
bumped into that group on login. In addition, the user is removed from all
local groups that are not present in the list of groups provided by Keycloak.

Please note that this breaks local groups as users will be removed from all
local groups they are not in as well on the Keycloak server.