Turn on thumbnail support for `attachment:audio` and `attachment:video`. Change conditionals to check for theme OR post type support when determining whether to turn on the Featured Image UI pieces in the admin.

Fixes #27460.


Built from https://develop.svn.wordpress.org/trunk@27657


git-svn-id: http://core.svn.wordpress.org/trunk@27500 1a063a9b-81f0-0310-95a4-ce76da25c4cd