Convert bugzilla email back into username when possible
    
    Basically we have a mapping username -> email, so we can build the
    opposite one email -> username which we can then use to try to translate
    bugzilla email back into FAS accounts.
    
    This assumes bugzilla_email is unique (which it should).
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>