Per-user libraries

This will be much more powerful than the current per-instance library, for various reasons:

1. Users will have a configurable quota, and be able to upload content themselves, without needing specific permissions. This mean anybody can enjoy its own music on Funkwhale
2. Users will decide if they want to share their libraries with their instance, followers, or keep those private (recommended for libraries that contain copyrighted content)
3. It will be easier for creators to share their content on the network
4. Instance owners will have more incentive to open their instance to signup, since users will be responsible for what is uploaded, and you can easily disable a user account or library

Possible issues with the change:

1. Performance, as we have an intermediary level to manage (users -> libraries -> tracks)
2. Metadata issues, since multiple users may upload the same tracks in different libraries, but we have to link them as the same track
3. Increased load on instances
4. Migration path (instance-level libraries to user-level libraries)