allow unrecognized outputs and cell types in notebooks from the future

in nbformat schema / validation
Handle unrecognized outputs and cells from the future
First stab at ConfigManager class
Add default for profile_dir

So ConfigManager is useful outside an IPython application
Remove profile_dir from handler class
Preserve nbformat_minor from the future

warn on click of unrecognized elements, rather than on load.
close pager on escape on command mode
Pin the version of less used in the Dockerfile.
Update package name in error prompt.
Merge pull request #6991 from minrk/unhandled-types

Handle unrecognized output and cell types
Merge pull request #7070 from rgbkrk/pin_less_docker

Pin the version of less used in the Dockerfile
Don't calculate default value when setting traitlet for the first time
Merge pull request #7041 from takluyver/nbconfig-manager

ConfigManager class for frontend config
Merge pull request #7068 from Carreau/close-pager

close pager on escape on command mode
use less variable for rounded corner

and allow none in versions comparison