Feature: Implement extended multi-user handling

[faradox]

As a Nendo Administrator, I would like to allow multiple users to share a library. Currently, every user has their own library, completely isolated from each other. However, in some cases, I would prefer to have the ability to share certain items from my library with other users.

Action items:

• Allow setting a track's / collection's visibility to public, in which case all users will see the collection
• Add a selector in the frontend to switch between seeing "my collections" and "all collections"
• Add a new UAC table to nendo-server that allows to map private collections to user_id's who are permitted to see the respective collection
• Change the /tracks route to accept a parameter that allows to switch between viewing all tracks that a user created themselves and viewing all tracks to which the user has access permissions
• Also add a filter/switch for that parameter to the frontend
• Adjust all track/collection modification and deletion routes to only allow a user to edit or delete tracks/collections that are directly owned by the user, not those created by other users which he is permitted to view

[samim23]

A related question here is, if there should be a mechanism to "subscribe" to a users entire libraries (VS just collections) which then show up in my library in a unified interface.

Generally speaking, closely related to this ticket, is the question of the access control model