Customize: Make sure the FTP credentials modal is visible.

The customizer and the theme installer use `visibility: hidden` on the body when
they open full-overlays screens. The FTP credentials modal needs a visibility
property set back to `visible` to be visible over those overlays.

Props purnendu.
Fixes #42205.

Built from https://develop.svn.wordpress.org/trunk@41972


git-svn-id: http://core.svn.wordpress.org/trunk@41806 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/forms-rtl.css

@@ -952,6 +952,8 @@ table.form-table td .updated p {
 
 .request-filesystem-credentials-dialog {
 	display: none;
+	/* The customizer uses visibility: hidden on the body for full-overlays. */
+	visibility: visible;
 }
 
 .request-filesystem-credentials-dialog .notification-dialog {