Validate that the username is alphanumeric and is not already in use.
        Also validates that the username is not listed in
        ACCOUNTS_FORBIDDEN_USERNAMES list.

def clean_username(self):
        """
        Validate that the username is alphanumeric and is not already in use.
        Also validates that the username is not listed in
        ACCOUNTS_FORBIDDEN_USERNAMES list.
        """
        try:
            get_user_model().objects.get(
                username__iexact=self.cleaned_data['username'])
        except get_user_model().DoesNotExist:
            pass
        else:
            raise forms.ValidationError(_('This username is already taken.'))
        if self.cleaned_data['username'].lower() in accounts_settings.ACCOUNTS_FORBIDDEN_USERNAMES:
            raise forms.ValidationError(_('This username is not allowed.'))
        return self.cleaned_data['username']