When we generate links from URL's in messages, we add rel="nofollow" to combat link spam. This change turns that into rel="nofollow noopener". This prevents the page at the end of the link from having access to the roundup window that displays the link. Details on the issue are are at: https://mathiasbynens.github.io/rel-noopener/ search web for noopener vulnerability. This problem usually requires a target="_blank" to really exploit it and we don't provide that. But adding noopener is extra protection.

Docs
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.. toctree::
   :maxdepth: 2

   docs/features
   docs/installation
   docs/upgrading
   docs/FAQ
   docs/user_guide
   docs/customizing
   docs/admin_guide
   docs/xmlrpc
   Design (original) <docs/design>
   docs/developers
   docs/tracker_templates
   docs/acknowledgements
   docs/license