Restore disquscdn.com to yellowlist

As mitigation until the MDFP list for Disqus is distributed (1e9f217).

Also to help users who cookieblock or allow the *.disqus.com domain, but
don't think to move the slider for disquscdn.com domains.

Following up on #2613

src/data/yellowlist.txt

@@ -138,6 +138,7 @@ digitalriver.com
 digitalrivercontent.net
 cdn.discourse.org
 discovery.com
+disquscdn.com
 dmcdn.net
 dmcdn.com
 screendoor.dobt.co