make bug disappear:

```django.core.management.base.CommandError: System check identified some issues:

ERRORS:
feedback_form.Feedback.user: (fields.E301) Field defines a relation with the model 'auth.User', which has been swapped out.
	HINT: Update the relation to point at 'settings.AUTH_USER_MODEL'.

System check identified 1 issue (0 silenced).
```
add missing dependency