Intelligently make media fields readonly when the user cannot update or do...

Intelligently make media fields readonly when the user cannot update or do anything with them. props koopersmith, helenyhou, nacin. fixes #22613

git-svn-id: https://core.svn.wordpress.org/trunk@23072 1a063a9b-81f0-0310-95a4-ce76da25c4cd