Courses which one has already joined should not be displayed in selection dialog when joining a school class.

[Jonas-Sander]

Describe the bug

When joining a school class one can choose which courses (that are linked to the class) to join.
This list of courses should not include courses which the user has already joined (before joining the school class).

Steps to reproduce the bug

Steps to reproduce the behavior: (See videos below)

1. [Account A] Go to groups page and create a school class called "8a".
2. [Account A] Tap on to "8a" class.
3. [Account A] Create two new courses (e.g. maths and english) that are linked with the "8a" class.
4. [Account A] Copy down the sharecode of the "8a" class.
5. [Account A] Copy the sharecode of one of the courses (e.g. maths).
6. [Account A] Log out.
7. Log In with Account B.
8. [Account B] Join the maths course via the sharecode from above.
9. [Account B] Start joining the class via the sharecode from above.

Current broken behavior

The course selection dialog includes already joined courses (in the example above maths).
It doesn't matter if you've selected the course or not to join (not selecting it in the dialog does not mean you leave the course if you've already joined it before).

Expected behavior

All courses which you've already joined before joining the class should be left out of the selection dialog.

Screenshots, videos or logs

(See step-by-step explanation from above.)

Account A:

https://user-images.githubusercontent.com/29028262/152443702-9c0adc17-31f6-437c-a8f3-2690ddcc36aa.mp4

Account B:

https://user-images.githubusercontent.com/29028262/152443717-f7487a35-bcbb-4d75-b97d-5406e718eaa3.mp4

Tested device

• Device: Pixel 4a
• OS: Android 12
• App type: Android app
• App version: 1.6.1 (315)

Additional context

Original issue: https://gitlab.com/codingbrain/sharezone/sharezone-app/-/issues/1251