Media Grid: add Bulk Selection mode for deleting attachments.

* Toolbar is sticky when `select` mode is active
* Selection is toggled when clicking an attachment preview
* Unselected attachments fade out, selected fade in.

See #28842.

Built from https://develop.svn.wordpress.org/trunk@29484


git-svn-id: http://core.svn.wordpress.org/trunk@29262 1a063a9b-81f0-0310-95a4-ce76da25c4cd