#4644 Don't whitelist form elements in markdown processing
<textarea> is whitelisted, but pretty useless (and surprising) to see rendered as a real textarea. There doesn't seem to be a use for any form element to be rendered.
Our HTMLSanitizer preprocessor uses feedparser._HTMLSanitizer. We could subclass that to remove items from acceptable_elements.
It would be nice if these were automatically escaped, rather than removed.
On branch db/4644
To test, start on the master branch and create a comment/page/ticket/anything using form tags. Then switch to this branch and that existing content (as well as any new posts) will escape the form tags.