Replace html5lib sanitizer

[twm]

html5lib has deprecated its sanitizer. Decide on a course of action. Some possibilities:

• Vendor html5lib's sanitizer. This is pretty much the status quo, since Yarrharr pins html5lib and monkeypatches the sanitizer (and serializer!) to fix issues (
• Replace the sanitizer with Bleach, as discussed here. Yarrharr would still need to patch the serializer.
• Replace html5lib entirely, since it's unmaintained for all meaningful values of "maintained".