Stored XSS via Kroki diagram

If Kroki has been enabled, it's possible to craft a `pre` block so that arbitrary attributes can be injected into the resulting `img` tag.

        diagram_selectors = ::Gitlab::Kroki.formats(settings)
                                .map do |diagram_type|
                                  %(pre[lang="#{diagram_type}"] > code,
                                  pre > code[lang="#{diagram_type}"])
                                end …