Fix incorrect target for user notifications

[carlobortolan]

Motivation and Context

See issue #1298.

Description

Included target field to notification model's JSON.

Steps for Testing

Prerequisites:

• 1 admin, 1 student
• Optionally also 1 lecturer or loggedIn user

1. Log in as an admin
2. Open the notifications panel (http://localhost:8081/admin/notifications) and create a few new notifications with a specific target
3. Log out and log in again using a different user to see the correct notifications (e.g., now a student-user should only see notifications with targets: Students and All Users)

Screenshots

[github-actions]

Your Testserver will be ready at https://1397.test.live.mm.rbg.tum.de in a few minutes.

Logins

Kurs1	Kurs2	Kurs3	Kurs4
public	public	loggedin	enrolled
prof1	prof1	prof2	prof1 prof2
student1 student2 student3	student1 student2	student2 student3	student1 student2