Fixed login form to reference USERNAME_FIELD

Was hardcoded to say 'username' when USERNAME_FIELD may be 'email' etc.
That won't work for some languages - e.g. in German the word for "your" differs according to the gender of the thing after it. I'm a bit hazy about how translation string compilation works, but if `ugettext_lazy("Enter your %s" % get_user_model().USERNAME_FIELD)` ends up hard-coding "Enter your username" as the translation (meaning that people with non-standard username fields don't get a translation), I'd rather do that than give our translators ambiguous strings.