Get users and user profiles

When the user profiles are in place, add different endpoints to get user profiles and user models.

- [ ] Implement `/users/profile` to get user profiles of all available users. This should be accessible to all users.
- [ ] Implement `/users` to get all user models. The endpoint should be accessible only to admin users.
- [ ] Implement `/users/<user-id>` to get a user matching the given ID.