Fixed login form to reference USERNAME_FIELD

Was hardcoded to say 'username' when USERNAME_FIELD may be 'email' etc.
Hi all,

@gasman is correct in assuming that this is a problem for languages like German which have grammatical genders and conjugate the "your" accordingly. So in this example, two possible output strings would be "Geben Sie Ihren Benutzernamen ein!" and "Geben Sie Ihre Emailadresse ein!". See the difference between "Ihren"/"Ihre" (because Benutzername is grammmatically male and Email is grammatically female, obviously! :-). And I don't think you can prepare for this eventuality without allowing different translations for the complete sentence.
However, there are ways around the problem. As @kaedroho mentioned, it might be best for the German translation to rearrange this to a sentence without gender connotations like "Bitte %s eingeben", which is the same for both genders.

In the end, in this instance I think it would be best to select a form that is best for the english strings and add a comment that %s can be username, email, ..., so the translators know they have to keep it gender-neutral. Dynamic text composition is always going to be hard on the translators, however.