Show Insert into post button in the media library when the current post...

Show Insert into post button in the media library when the current post supports editor (not the attachment parent), props SergeyBiryukov, fixes #18912

git-svn-id: https://core.svn.wordpress.org/trunk@19350 1a063a9b-81f0-0310-95a4-ce76da25c4cd