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/structures/UserSettings.js

@@ -605,7 +605,7 @@ module.exports = React.createClass({
             <div>
                 <h3>Referral</h3>
                 <div className="mx_UserSettings_section">
-                    { _t("Refer a friend to Riot:") } <a href={href}>{ href }</a>
+                    { _t("Refer a friend to Riot:") } <a href={href} target="_blank" rel="noopener">{ href }</a>
                 </div>
             </div>
         );