Set orig_nbformat keys when downgrading v4->v3
don't assume converted notebooks are old
    
in notebook.js dialogs, which can now be triggered
by nbformat 4 from the future.
include orig_nbformat_minor in v3 schema
Merge pull request #6844 from minrk/nbformat4-from-backport

a couple of nbformat 4 fixes I noticed while backporting v4 to 2.x