Set permissions after migrating

When setting up a new server and running migrations for the first time, you currently get warning about non-existent permissions. This is because `0004_create_user_groups.py` (in the `user` app) tries to add permissions to user groups while the permissions themselves do not exist. Apparently, Django only creates these after running migrations (in the `post_migrate` signal, using `create_permissions`, cf. `django.contrib.auth.apps.AuthConfig`).

You can get around this by undoing and redoing the last migration

```sh
python manage.py migrate user 0003
python manage.py migrate user
```
Needless to say this is not a very nice setup. To solve this we could write a separate, idempotent script that is run after all migrations have been run and that sets all group permissions according to the overview in `backend/user/permissions.py`.