Added Filter by Role to user search

[SebiWrn]

Motivation and Context

You can now filter users by their role, improves administration and overview over admins.

Steps for Testing

Prerequisites:

• 1 Admin

1. Log in
2. Navigate to User management page
3. View the new Dropdown

Screenshots

[github-actions]

Your Testserver will be ready at https://1278.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

[YiranDuan721]

There's a small problem: after filtering, the user's email doesn't seem to be displayed