replace hard-coded reference to user model...

... from django.contrib.auth.models.User to django.conf.settings.AUTH_USER_MODEL

e.g. when running into the following error after upgrading sentry:
```
CommandError: One or more models did not validate:
sentry_responsible.responsibility: 'user' defines a relation with the model 'auth.User', which has been swapped out. Update the relation to point at settings.AUTH_USER_MODEL.
```