Fix false order of videos on course-page

[MatthiasReumann]

Motivation and Context

Resolves #1143

Description

Negates predicate which determines order of groups, i.e. months.

Steps for Testing

• 1 Course

1. Log in
2. Navigate to a Course
3. Re-order Elements
   1. Oldest first: Oldest month first; Oldest stream first;
   2. Newest first: Newest month first; Newest stream first;
4. 🌟

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Your Testserver will be ready at https://1145.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 |