Merge Security Pull Request: google-caja...
Merge Security Pull Request: google-caja
Adds HTML sanitization.
The basics:
- untrusted HTML is always sanitized, with no warning
(there is console logging for changes made)
- markdown is always treated as untrusted
- no warnings for simply excluded output (e.g. Javascript)
- CSS tags and attributes are always stripped from untrusted HTML
- never check whether HTML is "safe," only sanitize
- add 'Trust notebook' to File menu