Media: Expose media grid frame via `wp-media-grid-ready` trigger for easier customization.

Props soulseekah.
Fixes #50185.
Built from https://develop.svn.wordpress.org/trunk@47807


git-svn-id: https://core.svn.wordpress.org/trunk@47583 1a063a9b-81f0-0310-95a4-ce76da25c4cd