[1.11.x] Fixed #28645 -- Reallowed AuthenticationForm to raise the inactive user error when using ModelBackend.

      Regression in e0a3d937309a82b8beea8f41b17d8b6298da2a86.

Thanks Guilherme Junqueira for the report and Tim Graham for the review.

Backport of 359370a8b8ca0efe99b1d4630b291ec060b69225 from master