Don't show the Featured Image metabox for users with the `contributor` role. Do enqueue media so that TinyMCE previews work for YouTube embeds, etc.

Props SergeyBiryukov.
Fixes #28327.

Built from https://develop.svn.wordpress.org/trunk@29402


git-svn-id: http://core.svn.wordpress.org/trunk@29180 1a063a9b-81f0-0310-95a4-ce76da25c4cd