refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/layouts/sidebar.js

@@ -0,0 +1,7 @@
+import { modeWatcher } from '../components/mode-watcher';
+import { sidebarExpand } from '../components/sidebar';
+
+export function initSidebar() {
+  modeWatcher();
+  sidebarExpand();
+}