Allow formatting and cleaning up usernames the same way as in the LDAP and OIDC modules

Usernames are currently cleaned by replacing forbidden characters with underscores, but this is not consistent with how the LDAP and OIDC extensions clean up usernames. We introduce a userFormat parameter which works like the existing groupFormat parameter, and use the same code as in the LDAP and OIDC extensions for both. Using this code is for groupFormat is possible because it was designed to be compatible with this way of cleaning names.