propagate display metadata to all mimetypes

Additionally, this PR removes in-memory short keys in favor of using mimetype keys within the notebook javascript code, and tests toJSON/fromJSON codepaths for all of the supported mimetypes.

- [x] text/plain
- [x] text/html
- [x] image/svg+xml
- [x] image/png
- [x] image/jpeg
- [x] text/latex
- [x] application/json
- [x] application/javascript
- [x] some garbage mimetype