forms.py: change NextUrlFormMixin's field "next_url" to be optional (fixes #6917)

If not optional and no next_url parameter is given, an error is raised
and the user can never complete the form.

src/authentic2/forms.py

@@ -109,7 +109,7 @@ class EmailChangeForm(forms.Form):
         return password
 
 class NextUrlFormMixin(forms.Form):
-    next_url = forms.CharField(widget=forms.HiddenInput())
+    next_url = forms.CharField(widget=forms.HiddenInput(), required=False)
 
     def __init__(self, *args, **kwargs):
         from .middleware import StoreRequestMiddleware