feat(back): Return units to a course

Add more data to the user endpoint. The user does now give the unit ids connected to each enrollment togheter with its date.

To test this run the /user endpoint. This is implemented so that the user can get a visual representation of how many units are unanswered to a unit. For overview info page.

Note:

• It also filters out:
  • units that are in the future
  • units that are answered