feat(frontend) Connect create/edit/delete unit to backend (#58)

• Add create unit function that sends a POST request to the backend
• Add edit unit function that sends a PATCH request to the backend
• Add delete unit functionality that sends a DELETE request to the backend
• Add backend endpoint for updating Unit
• Add backend endpoint for deleting unit
• Fix small bugs in frontend

HOW TO TEST:

1. Try to create new unit as a lecturer and it should appear on the CourseOverview page.
2. Then try to edit the unit (need to be under upcoming units to be able to edit), and the unit should be updated.
3. Then try to delete the unit by clicking on either open/edit unit and then delete unit.

Closes #58 (closed)