refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/components/locale-datetime.js

@@ -0,0 +1,50 @@
+/**
+ * Update month/day to locale datetime
+ *
+ * Requirement: <https://github.com/iamkun/dayjs>
+ */
+
+/* A tool for locale datetime */
+class LocaleHelper {
+  static get attrTimestamp() {
+    return 'data-ts';
+  }
+
+  static get attrDateFormat() {
+    return 'data-df';
+  }
+
+  static get locale() {
+    return $('html').attr('lang').substring(0, 2);
+  }
+
+  static getTimestamp(elem) {
+    return Number(elem.attr(LocaleHelper.attrTimestamp)); // unix timestamp
+  }
+
+  static getDateFormat(elem) {
+    return elem.attr(LocaleHelper.attrDateFormat);
+  }
+}
+
+export function initLocaleDatetime() {
+  dayjs.locale(LocaleHelper.locale);
+  dayjs.extend(window.dayjs_plugin_localizedFormat);
+
+  $(`[${LocaleHelper.attrTimestamp}]`).each(function () {
+    const date = dayjs.unix(LocaleHelper.getTimestamp($(this)));
+    const text = date.format(LocaleHelper.getDateFormat($(this)));
+    $(this).text(text);
+    $(this).removeAttr(LocaleHelper.attrTimestamp);
+    $(this).removeAttr(LocaleHelper.attrDateFormat);
+
+    // setup tooltips
+    const tooltip = $(this).attr('data-toggle');
+    if (typeof tooltip === 'undefined' || tooltip !== 'tooltip') {
+      return;
+    }
+
+    const tooltipText = date.format('llll'); // see: https://day.js.org/docs/en/display/format#list-of-localized-formats
+    $(this).attr('data-original-title', tooltipText);
+  });
+}