refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/layouts/topbar.js

@@ -0,0 +1,9 @@
+import { convertTitle } from '../components/convert-title';
+import { displaySearch } from '../components/search-display';
+import { switchTopbar } from '../components/topbar-switcher';
+
+export function initTopbar() {
+  convertTitle();
+  displaySearch();
+  switchTopbar();
+}