nbformat v4

Still quite a bit to do, but the implemented parts of v4 are working pretty well. I did find and fix some things while testing the validation. Both v3 and v4 have a Draft 4 jsonschema, and use the standard jsonschema format for references, so jsonpointer is no longer used.

closes #5074 

TODO in separate PRs:

- backport v4->v3 to 2.x
- what to do when uploading old notebooks (upgrade on upload, or leave alone and upgrade on open)?


- [x] add convert 3<->4
- [x] make v4 current
- [x] update stream messages to `name, text`
- [x] move display_data.png to display_data.data['image/png']
- [x] what's new
- [x] decide on UI for downgrade
- [x] upgrade existing example notebooks
From discussion in person: currently, NotebookNotary hardcodes details of the v4 format, so we cannot now sign v3 notebooks. We think we should probably be able to sign v3 and v4 notebooks, so that third party code creating v3 notebooks is able to sign them.