refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/components/sidebar.js

@@ -0,0 +1,25 @@
+/**
+ * Expand or close the sidebar in mobile screens.
+ */
+
+const $body = $('body');
+const ATTR_DISPLAY = 'sidebar-display';
+
+class SidebarUtil {
+  static isExpanded = false;
+
+  static toggle() {
+    if (SidebarUtil.isExpanded === false) {
+      $body.attr(ATTR_DISPLAY, '');
+    } else {
+      $body.removeAttr(ATTR_DISPLAY);
+    }
+
+    SidebarUtil.isExpanded = !SidebarUtil.isExpanded;
+  }
+}
+
+export function sidebarExpand() {
+  $('#sidebar-trigger').on('click', SidebarUtil.toggle);
+  $('#mask').on('click', SidebarUtil.toggle);
+}