Send nocache_headers() on installation screens and when redirecting to them.

fixes #29248.
Built from https://develop.svn.wordpress.org/trunk@29599


git-svn-id: http://core.svn.wordpress.org/trunk@29373 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Media Grid: exit selection mode when Esc is pressed.

Fixes #29366.

Built from https://develop.svn.wordpress.org/trunk@29600


git-svn-id: http://core.svn.wordpress.org/trunk@29374 1a063a9b-81f0-0310-95a4-ce76da25c4cd