CVE-2021-43827 Inline footnotes wrapped in <a> tags can cause errors in discourse-footnotes

discourse-footnote is a library providing footnotes for posts in Discourse. ### Impact When posting an inline footnote wrapped in <a> tags (e.g. <a>^[footnote]</a>, the resulting rendered HTML would include a nested <a>, which is stripped by Nokogiri because it is not valid. This then caused a javascript error on topic pages because we were looking for an <a> element inside the footnote reference span and getting its ID, and because it did not exist we got a null reference error in javascript. Users are advised to update to version 0.2. As a workaround editing offending posts from the rails console or the database console for self-hosters, or disabling the plugin in the admin panel can mitigate this issue.