contribute.jquery.org: add CSP exception for vimeo player

Ref jquery/infrastructure-puppet#54

themes/contribute.jquery.org/functions.php

@@ -0,0 +1,8 @@
+<?php
+
+// Allow loading a Vimeo video on
+// https://local.contribute.jquery.org/markup-conventions/
+add_filter( 'jq_content_security_policy', function ( $policy ) {
+	$policy[ 'frame-src' ] = "'self' player.vimeo.com";
+	return $policy;
+} );