Store licencing information about tracks/releases

This would help avoiding federating copyrighted music or serving them publicly.

Interesting resources about the subject:

• https://mastodon.eliotberriot.com/@eliotberriot/101150980790498722
• http://musique-libre.org/doc/le-tableau-des-licences-libres-et-ouvertes-de-dogmazic/
• https://spdx.org/licenses/
• https://www.gnu.org/licenses/licenses.html