Sanitizer: drop all CSS

[twm]

The current CSS sanitizing regime has a lot of issues:

• CSS Sanitizer "gauntlet" filters style tags like font-family:"sans-serif" html5lib/html5lib-python#69
• thin and thick not in CSS whitelist html5lib/html5lib-python#75
• sanitize_css sort of ignores allowed_css_properties html5lib/html5lib-python#317
• sanitize_css gauntlet rejects valid css html5lib/html5lib-python#316

Most importantly though, it doesn't parse CSS. It's doubtful that the implementation is correct, and therefore, secure. For now, the safest thing to do is to drop all CSS.