Site Logo: Add a media state for site logos in the Media Manager.
Site Logo: Update the Customizer section heading to improve discoverability.

Also simplified the control name to just "Logo" (keeps it concise and matching the section heading).
Site Logo: Use the proper helper function for determining registered image sizes.

This will break on WP.com, as `get_intermediate_image_sizes()` has been neutered because of Photon. We'll deal with it over there.