Accessibility: Media: Add a "Copy URL" button to the attachment File URL fields.

For a number of years, various screens in the WordPress admin provided users with a readonly input field to copy the attachment file URL. Manually copying from a readonly field is an annoying task at best even for mouser users. It's a usability and accessibility issue at the same time. 
These fields now have a new "Copy URL" button that is easy to use and accessible to everyone.

Props theolg, markdubois, vabrashev, sajjad67, xkon, nrqsnchz, melchoyce, audrasjb, afercia.
See #41612, #50322, #50335.
Fixes #48463.

Built from https://develop.svn.wordpress.org/trunk@48232


git-svn-id: https://core.svn.wordpress.org/trunk@48001 1a063a9b-81f0-0310-95a4-ce76da25c4cd