Upgrade HTML Purifier to latest version

Test 1 Log in as Admin. Create a Course. Enrol a Student in the Course. Create a Forum in the Course. Log in the Student. Go to the Forum. Create a discussion. In the discussion message, add a couple of HTML links with "Open in new window" checked and a couple with "Open in new window" unchecked. Save. View the discussion and verify , viewing source code, that any links that have the " target="xxxx" " attribute will have " rel=noreferrer noopener " attribute added. Make sure  when opening a target="_blank" link that the original page is not redirected and the link is open in a new tab/window. Try the same sort of thing in other text areas and see if any don't add the attribute. Example: This: <a href="http://allthethings.co.nz/moodle/redirect.html" target="_blank">Test my link</a>   Becomes: <a href="http://allthethings.co.nz/moodle/redirect.html" target="_blank" rel="noreferrer noopener">Test my link</a>   Test 2 (No manual test is required here): Run all unit tests and check that failures don't relate to this issue.