Add target="_blank" to links that don't have it

This is now more of a problem in the Electron app because of
https://github.com/electron/electron/issues/8841 but is still
annoying in the webapp if you're taken away from your chat client.

Exception is the SSO link, as commented (issue filed at
https://github.com/vector-im/riot-web/issues/8247).

Fixes https://github.com/vector-im/riot-web/issues/8226

src/components/views/auth/AuthFooter.js

@@ -26,7 +26,7 @@ module.exports = React.createClass({
     render: function() {
         return (
             <div className="mx_AuthFooter">
-                <a href="https://matrix.org">{ _t("powered by Matrix") }</a>
+                <a href="https://matrix.org" target="_blank" rel="noopener">{ _t("powered by Matrix") }</a>
             </div>
         );
     },