Refactor assets bundler and split Javascript files

ui/static/js/modal_handler.js

@@ -0,0 +1,31 @@
+class ModalHandler {
+    static exists() {
+        return document.getElementById("modal-container") !== null;
+    }
+
+    static open(fragment) {
+        if (ModalHandler.exists()) {
+            return;
+        }
+
+        let container = document.createElement("div");
+        container.id = "modal-container";
+        container.appendChild(document.importNode(fragment, true));
+        document.body.appendChild(container);
+
+        let closeButton = document.querySelector("a.btn-close-modal");
+        if (closeButton !== null) {
+            closeButton.onclick = (event) => {
+                event.preventDefault();
+                ModalHandler.close();
+            };
+        }
+    }
+
+    static close() {
+        let container = document.getElementById("modal-container");
+        if (container !== null) {
+            container.parentNode.removeChild(container);
+        }
+    }
+}