Media: Disable lazy-loading for custom logos by default.

Custom site logos are most commonly displayed above the fold, so lazy-loading is unnecessary.

Props demetris, khag7, johnbillion, rebasaurus, audrasjb.
Merges [48870] to the 5.5 branch.
Fixes #50933.
Built from https://develop.svn.wordpress.org/branches/5.5@48871


git-svn-id: https://core.svn.wordpress.org/branches/5.5@48633 1a063a9b-81f0-0310-95a4-ce76da25c4cd