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.
On the client side:
- Makes the syntax_highlight JS more flexible
- Adds JS specs
- Simplifies highlighting of new notes
- Adds highlighting to Markdown preview