Syntax highlighting improvements
On the server side:
During development I would occasionally see SanitizationFilter sanitizing
the result of SyntaxHighlightFilter, even though its attributes were
whitelisted. This updates the `clean_spans` transformer to return the
whitelisted node as [suggested by the Sanitize docs](http://git.io/vZR8i).
On the client side:
- Makes the syntax_highlight JS more flexible
- Adds JS specs
- Simplifies highlighting of new notes
- Adds highlighting to Markdown preview
See merge request !1278
