Customize: Load the default site icon from the `wp-includes` directory.

Files inside the `wp-admin` directory may not be publicly available.

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

Props whyisjake, finomeno, ocean90.
Fixes #50131.
Built from https://develop.svn.wordpress.org/trunk@47832


git-svn-id: https://core.svn.wordpress.org/trunk@47608 1a063a9b-81f0-0310-95a4-ce76da25c4cd