[#7528] change html sanitization library to fix XSS issue

Feedparser library hasn't been updated in years.  html5lib
has the same sanitizer with updates, including fixes for this parsing
issue.  It also has other behavioral changes including:

* trailing slashes removed from URLs (weird, and not ideal)
* quotes around attributes like href removed when not necessary
* changed order of attributes
* no closing </p>; <br> instead of <br />; etc
* disallowed tags are escaped instead of removed
* comments are removed instead of left in