Media grid: Don't show a misleading message for an empty library.

This uses the existing string from the list table. We should revisit a better overall experience for the two situations (empty library vs. search with no results) in 4.1.

props voldemortensen.
fixes #29404.

Built from https://develop.svn.wordpress.org/trunk@29655


git-svn-id: http://core.svn.wordpress.org/trunk@29429 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Editor expand:
- Adjust calculation when comparing sidebar position to wpfooter top. Fixes an annoying jumping when the user scrolls off the bottom.
- Use a var for the 'bottom' distance of the sidebar.
- Fix sidebar pinning when it's height is nearly the same as the viewport height.
Fixes #29349
Built from https://develop.svn.wordpress.org/trunk@29656


git-svn-id: http://core.svn.wordpress.org/trunk@29430 1a063a9b-81f0-0310-95a4-ce76da25c4cd