Add support for basic HTML tags in comments

Use bleach (an awesomely named, awesome python lib) to remove any HTML
tags and attributes that are not explicitly white-listed.  Supported
tags: a, abbr, acronym, b, blockquote, code, em, i, li, ol, strong, ul,
img.  HTML strip is done server side, both when comments are created,
and when they're pulled from the db (to protect end users against a
corrupt db).