Multiple fixes

- Fixed pwa location in manifest.json
- made database generator check the .env for location rather than runtime attribute
- fixed dark mode preference based on cookie/browser preference
- changed to socketio.sleep for compatibility

Server/static/manifest.json

@@ -2,7 +2,7 @@
     "name": "Jukebox Remote",
     "short_name": "Jukebox Remote",
     "description": "Controller for the PartyJukebox server app.",
-    "start_url": "index.html",
+    "start_url": "/",
     "display": "standalone",
     "background_color": "#eeeeee",
     "theme_color": "#eeeeee",