Implement upload throttling: one file at a time.

public/css/default.css

@@ -697,6 +697,11 @@ a#previous_photo:hover, a#next_photo:hover {
 #upload_preview_area .progress .error {
 	background: #b22;
 }
+#photo_submit:disabled {
+	background: #ccc;
+	color: #777;
+	border-color: #aaa;
+}
 
 /* Spinner animation
 ----------------------*/