Customize: Move the WordPress logo with a white background to the `wp-includes` directory.

This ensures that the image used as a default site icon looks good on a dark background.

The image was previously changed in the `wp-admin` directory, but the site icon is now loaded from `wp-includes`.

Follow-up to [36635], [47018], [47564], [47832].

Props ocean90.
Fixes #49798.
Built from https://develop.svn.wordpress.org/trunk@47838


git-svn-id: https://core.svn.wordpress.org/trunk@47614 1a063a9b-81f0-0310-95a4-ce76da25c4cd