frontend: accept admin permissions for Copr build
    
    As discussed in https://github.com/packit/packit-service/issues/1136,
    admins of Copr repositories still need to have approved `builder`
    permissions for submitting Copr builds.
    
    Change the behavior to accept admin privileges too without a need to
    explicitly grant the permissions either manually or automatically.
    
    Fixes #1951
    
    Signed-off-by: Matej Focko <mfocko@redhat.com>