cleanup: Stop using string concatenation

It's unfortunate that we cannot fully embrace template strings, but
we aren't limited to "legacy" formatting either; replace the last
remaining places where we still use string concatenation to reduce
the difference between regular and legacy style a bit.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2200>

js/ui/components/telepathyClient.js

@@ -816,7 +816,8 @@ var ChatNotification = HAVE_TP ? GObject.registerClass({
 
         /* Translators: this is the other person changing their old IM name to their new
            IM name. */
-        let message = '<i>' + _("%s is now known as %s").format(oldAlias, newAlias) + '</i>';
+        const message = `<i>${
+            _('%s is now known as %s').format(oldAlias, newAlias)}</i>`;
 
         this._append({ body: message,
                        group: 'meta',