wpLink: fix accessibility issues:

- Add role=dialog and an aria-labelledby attribute to the modal container.
- Change the modal title to h1.
- Remove one ` ` that gets read out as `blank` by screen readers.
- Replace the Cancel link with a button.
- Clean up unused CSS.

Props afercia.
Fixes #30468.
Built from https://develop.svn.wordpress.org/trunk@36991


git-svn-id: http://core.svn.wordpress.org/trunk@36958 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/wplink.js

@@ -35,7 +35,7 @@ var wpLink;
 				wpLink.update();
 			});
 
-			inputs.close.add( inputs.backdrop ).add( '#wp-link-cancel a' ).click( function( event ) {
+			inputs.close.add( inputs.backdrop ).add( '#wp-link-cancel button' ).click( function( event ) {
 				event.preventDefault();
 				wpLink.close();
 			});