Add a new fixture: admin_user

This refactors the admin user out of the `admin_client` fixture and
provides it separately.

I have not used the `client` fixture with `admin_client` to keep them separate.
Maybe that should get done for `admin_user`, too?
User can be returned by ```create_superuser``` on previous line, I think that these three lines probably should be rewritten as:

```python
user = UserModel._default_manager.create_superuser('admin', 'admin@example.com',
                                                   'password', **extra_fields)
```

to reduce overall number of queries.