Remove Song.restricted field

[BeritJanssen]

Right now, we have an option to restrict playback of a section, defined on its associated song, to specific countries. We can actually remove this field altogether, as it's usually allowed to play copy-righted songs for academic purposes. This would mean the following:

• remove the field restricted from Song model and add a migration
• update the playlist.update_sections function and its test to not expect that csv field anymore
• update the hint under the Playlist csv TextField in the admin interface
• update the csv upload / download functionality accordingly
• remove all utility functions which make use of the restricted notion