panels: Rename "desktop_notifications_panel" => "panels".

The original code made a 3/4-hearted effort to generically accommodate
more banners/"panels" later, but named itself after the first one made.

[greg: expanded commit message.]

zproject/settings.py

@@ -1115,7 +1115,7 @@ JS_SPECS = {
             'js/ui_init.js',
             'js/emoji_picker.js',
             'js/compose_ui.js',
-            'js/desktop_notifications_panel.js'
+            'js/panels.js'
         ],
         'output_filename': 'min/app.js'
     },