Left menu logos update

The left menu icon key is deprecated and retained for backward compatibility. The icons key has been introduced to support separate light and dark fields for both themes:

{
...
  "left_menu": {
    ...
    "icons": {
      "light": "images/light.svg",
      "dark": "images/dark.svg"
    },
    ...
  }
  ...
}

Widget icons displayed in the left menu must now comply with the icon checklist for public integrations.