Fixed login form to reference USERNAME_FIELD

Was hardcoded to say 'username' when USERNAME_FIELD may be 'email' etc.
No, sorry. I meant "Enter your <field name here>"

The translation strings do not need to be literal. Translators are welcome to change and restructure sentences if it sounds better in the language they are translating into. I think "Enter your username" sounds best for English. "your" can be omitted from the German translation if the translator thinks it sounds better that way.