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[bot]]

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