Add an error message for inactive user login in AdminAuthenticationForm

[SeungWon Kang]

In admin login site, I found that ValidationError message for 'inactive' user does not exist. (In AuthenticationForm it exists) I know default BackEnd checks the is_active in user_can_authenticate() method, but I think adding this error message would be helpful if using other BackEnd like AllowAllUsersModelBackEnd.

In AuthenticationForm, it shows inactive error message to the inactive user(user that is_active=False)

    def confirm_login_allowed(self, user):
        ...
        if not user.is_active:
            raise forms.ValidationError(
                self.error_messages['inactive'],
                code='inactive',
            )

but in AdminAuthenticationForm, it shows invalid_login error message to the inactive user(user that is_active=False)

    def confirm_login_allowed(self, user):
        if not user.is_active or not user.is_staff:
            raise forms.ValidationError(
                self.error_messages['invalid_login'],
                code='invalid_login',
                params={'username': self.username_field.verbose_name}
            )

So I suggest to change it like

error_messages = {
        ...
        'inactive': _("This account is inactive."),
    }

    ...
    def confirm_login_allowed(self, user):
        if not user.is_active:
            raise forms.ValidationError(
                self.error_messages['inactive'],
                code='inactive',
            )

        if not user.is_staff:
            raise forms.ValidationError(
                self.error_messages['invalid_login'],
                code='invalid_login',
                params = {'username': self.username_field.verbose_name},
            )
    ....