Balancing sections over all sessions in an experiment

[BeritJanssen]

Is your feature request related to a problem? Please describe.
At this point, sections won't repeat within a session when using the get_unused_song_ids function. However, there is no balancing happening to make sure that sections which have been played fewer times over all sessions will have a higher chance to be picked.

Describe the solution you'd like
We can make use of the section.play_count in order to achieve such balancing (i.e., pick from the sections with the lowest play_count, but probably controlling at the same time for other section fields, such as tag, group, song-, artist- or filename)

Additional context
To make sure sessions from different blocks do not increase play counts for the same group of sections in an unexpected way, we may change experiment.playlist from a ManyToManyField to a ForeignKey on the Playlist side, essentially enforcing that each block define its own, unique playlist(s). Probably not so critical if documented well though.