Media grid bulk selection styling:

* Fade content within each attachment item so that focus styling is still clear.
* Less jarring toolbar pinning.
* Tighten up the fade in/out transition.

see #28842.

Built from https://develop.svn.wordpress.org/trunk@29489


git-svn-id: http://core.svn.wordpress.org/trunk@29267 1a063a9b-81f0-0310-95a4-ce76da25c4cd