Add a feature flag to show the dark mode UI toggle

[zackkrida]

This issue is part of the Dark Mode project: #3592. Please see the implementation plan for additional detail and context.

Description

Create a dark_mode_ui_toggle feature flag, which is off by default and switchable in staging. This will be used to conditionally render a new UI toggle for dark mode. You can view the designs here #4155.

The flag should have the following state:

{
   "dark_mode_ui_toggle": {
      "status": {
        "staging": "switchable",
        "production": "disabled"
      },
      "defaultState": "off",
      "description": "Display the UI toggle to change the site theme.",
      "storage": "cookie"
    }
}

This PR is only concerned with adding the flag, it does not need to implement the component.