Fix group order

[MatthiasReumann]

Motivation and Context

• Resolves #1149

Description

Dictionaries are not really great for ordering (i.e. they don't have a order). Rewrite the group function to return a 2-dimensional array (a array of stream groups starting in the same month and year).

Steps for Testing

• 1 Course

1. Log in
2. Navigate to the course page
3. Streams should now also be grouped correctly if multiple years occur. (e.g. 2022 December, 2023 January)

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Logins

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