Allow users to set a license for their tracks

[Manfred]

Sudara said, "I would like to add a license option to alonetone."

The default license for all tracks (assets in Alonetone) should be "All rights reserved", meaning they retain all the rights granted to then according to the laws in affect while uploading the audio.

Alternatively users can choose to change the license for a track (asset in Alonetone) to a Creating Common license by selecting it from a list of options. We could support other licenses in the future if the need arises.

We can also allow users to change the license for an entire album (playlist in Alonetone) by setting the license on the album. This will not be persisted in the database, but is forwarded to the tracks (assets).

Once of the considerations is that Creating Commons licenses have a version and that may change in the future. We will store the version select by the user, but should also be able to compare it to other versions.

As a follow-up to this issue we should write an issue about browsing tracks with a specific license.