FIX: Make discobot certificate faster/non blocking (#11344)

This moves the way we add the user avatar and site logo
to the discobot certificates from embeded base64 png to
just using the files urls in the href to the image tag.

This will make generation faster and the certificate
smaller overall, but it can't be used in a  `img` tag
anymore, since SVGs in `img` tags don't load the external images

In order to work around that we will move the certificate
in posts to an iframe, which works fine without any user
visible changes. For this to be possible the plugin automatically
adds the site current domain to the list of allowed iframe origins.

plugins/discourse-narrative-bot/lib/discourse_narrative_bot/base.rb

@@ -137,7 +137,7 @@ module DiscourseNarrativeBot
       src = Discourse.base_url + DiscourseNarrativeBot::Engine.routes.url_helpers.certificate_path(options)
       alt = CGI.escapeHTML(I18n.t("#{self.class::I18N_KEY}.certificate.alt"))
 
-      "<img class='discobot-certificate' src='#{src}' width='650' height='464' alt='#{alt}'>"
+      "<iframe class='discobot-certificate' src='#{src}' width='650' height='464' alt='#{alt}'>"
     end
 
     protected