ApostropheCMS: Mutation-XSS / allowedTags bypass via literal `</textarea/>` solidus close

Summary A mutation-XSS / allowedTags bypass: when textarea or xmp is included in allowedTags, an input containing a literal a solidus right after the RCDATA end-tag name lets non-allowed markup such as pass through sanitizeHtml live and unescaped, even though img/onerror are not in the allowlist....