Fixed login form to reference USERNAME_FIELD

Was hardcoded to say 'username' when USERNAME_FIELD may be 'email' etc.
Rather than guessing between ourselves about the most suitable string for the German translator to work with, perhaps we should ask @shezi directly... :-)

(@shezi, the start of the discussion is here: https://github.com/torchbox/wagtail/pull/705#discussion-diff-18942919)