Generic representation method for objects

We should have a generic way of representing any object in any format. The rough roadmap is:

• Provide a generic endpoint for finding a model (#871 (closed))
• Introduce a UUID for every object (#868)
• Make get_absolute_url return the generic URL for every object
• Add a generic method on ExtensibleModel to build a representation, e.g. render_as
  • Support for content negotiation using MIME types
  • MIME-type-to-renderer registry
  • Generic adherence to permissions/rules
  • Migrate GraphQL to use renderer system
  • Add text/html renderer that redirects to frontend URL (through a get_frontend_url method)
• Allow renderers to run in background
  • Revisit background task status notification (through HTTP status, etc.)

This allows a quite big set of use cases:

• Adding representations for JSON-LD, to start implementing ActivityPub
• Adding generic JSON APIs to objects
• Generically retrieve any object in a print version as PDF

assigned to @magicfelix

changed title from Recover concrete model instance by URI to Generic representation method for objects

changed the description