Make schema extensible

[bollwyvl]

Demo on Binder

This PR adds a GraphQLManager, a LoggingConfigurable which owns the schema and subscription serverr. At present, the schema extension is handled with entrypoints: this could be done with jupyter config, albeit round-aboutly, because of dict merging. The contents and fake subscription are added by default.

What is substantially missing is a compelling place yet for adding an extensible type and not just the top-level schema. Some places that could:

• Media
  • some media might have actually-extractable metadata, e.g. EXIF on images, so you'd like to be able to get at it.
• Notebook
  • an extension author might want to extend this for their purposes

Not sure the best route for this yet...