Allow updating of default_playlist_id

While implementing the default_playlist_id for schedules in the Dashboard I noticed that the logic in Steering was still missing. Since I needed it for my use case, I just added a very minimal implementation of the update logic.

While this might be okay for 0.99, it should probably be properly extended according to the TODO comment at some point

changed milestone to %Aura 0.9

requested review from @nnrcschmdt

mentioned in issue dashboard#65 (closed)

approved this merge request

mentioned in commit a191c32b

merged