backport nbformat v4 to 2.x
enable reading v4 notebooks

- add v4 to reader
- validation is not backported
- update some tests to reflect this
- test opening v4 notebooks
Set orig_nbformat keys when downgrading v4->v3
don't write orig_nbformat to disk in v3
don't assume converted notebooks are old

in notebook.js dialogs, which can now be triggered
by nbformat 4 from the future.
Merge pull request #6778 from minrk/2.x-nbformat-4

backport nbformat v4 to 2.x