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

wp-admin/includes/media.php

@@ -3268,6 +3268,10 @@ function attachment_submitbox_metadata() {
 	<div class="misc-pub-section misc-pub-attachment">
 		<label for="attachment_url"><?php _e( 'File URL:' ); ?></label>
 		<input type="text" class="widefat urlfield" readonly="readonly" name="attachment_url" id="attachment_url" value="<?php echo esc_attr( $att_url ); ?>" />
+		<span class="copy-to-clipboard-container">
+			<button type="button" class="button copy-attachment-url edit-media" data-clipboard-target="#attachment_url"><?php _e( 'Copy URL' ); ?></button>
+			<span class="success hidden" aria-hidden="true"><?php _e( 'Copied!' ); ?></span>
+		</span>
 	</div>
 	<div class="misc-pub-section misc-pub-filename">
 		<?php _e( 'File name:' ); ?> <strong><?php echo $filename; ?></strong>