use relative url for submodule
use https for submodule
use marked for markdown
remove pagedown
update components
use highlight.js to highlight markdown code blocks
remove prettify
unify highlight.js / codemirror style
Merge pull request #3264 from minrk/relative-submodule

use https url for submodule
finish removing prettify
Merge pull request #3101 from minrk/marked

use marked / highlight.js instead of pagedown and prettify
Moving handlers to subdir
Adding new files.
Splitting handlers into different files.

I have also created a top-level utils.py for notebook related stuff
Adding __init__.py
Fixing url join problems.
Removing debug message.