Course default playlist enhancements

This PR fixes #878,

Please refer to the linked-issue.

Things to consider:

A course must always have only one default playlist at a time.
Teachers must create a new playlist, or link already existing one as default playlist when the course doesn't have one.
There would be no more" Course playlist" or "Kurswiedergabeliste" in the sidebar, but a check icon with info.
A Playlist can be assigned to many courses as default one.
In Workspace Playlist table, those playlists that are default in courses will get an info icon with shows the courses.
The course titles will get truncated (...) if they are too long.
Teachers can change the default playlist of a course via the sidebar action item.
The title of the default course playlist can now be changed.
In case the default playlist is not created manually and the cronjob catches the course and recognizes that there is no default playlist, then it creates it automatically as before with the name of the course as playlist.

The following brief video is intended to help you with new changes.

elan-ev / studip-opencast-plugin