Show courses in video link to playliststs dialog

[dennis531]

Important Note:
Please merge after #893 as these changes would lead to conflicts.

Changes:

• Show info indicator with linked courses for each playlist
• Add header with course names to playlist dropdown

Problem:

• The same playlist is listed multiple times in the dropdown if it is linked to multiple courses. This could lead to unexpected behavior in the dropdown.
• Issue Default action for adding existing playlists to other courses should be copying - remove linking feature #886 should solve this issue, as a playlist can then only exist in one playlist

Screenshot:

Solve #794, #792
Fix #875

[github-actions]

This pull request has conflicts ☹
Please resolve those so we can review the pull request.
Thanks.

[tgloeggl]

#893 ist gemerged, magst du einmal die Merge conflicts für diesen PR hier resolven?

[dennis531]

#893 ist gemerged, magst du einmal die Merge conflicts für diesen PR hier resolven?

Done