Add noopener and/or noreferrer (#12202)

app/javascript/mastodon/features/ui/components/link_footer.js

@@ -62,7 +62,7 @@ class LinkFooter extends React.PureComponent {
           <FormattedMessage
             id='getting_started.open_source_notice'
             defaultMessage='Mastodon is open source software. You can contribute or report issues on GitHub at {github}.'
-            values={{ github: <span><a href={source_url} rel='noopener' target='_blank'>{repository}</a> (v{version})</span> }}
+            values={{ github: <span><a href={source_url} rel='noopener noreferrer' target='_blank'>{repository}</a> (v{version})</span> }}
           />
         </p>
       </div>