Table columns: dropdown/text filters per column

- tablekit builds a filter row under the headers for every th with data-filter;
  distinct cell values become a select (label from data-filter-label), or a
  text substring filter with data-filter="text"
- filters combine with the global table search; placeholder rows (colspan) are
  not filtered away; filter row follows the lab-focus column hiding
- enabled on: matrix lab columns (by variant), defense table (ЛР, status with
  readable values), students (Основа, Пароль), lab assignment lists (Уровень,
  Источник, Сдача), labs list (Статус)

web/src/main/resources/static/js/admin-group.js

@@ -104,7 +104,7 @@ function focusLab() {
     var s = $id('lab-focus');
     if (!s) return;
     var labId = s.value;
-    document.querySelectorAll('th.lab-col[data-lab], td.cell-lab[data-lab]').forEach(function (el) {
+    document.querySelectorAll('th.lab-col[data-lab], td.cell-lab[data-lab], .filter-row th[data-lab]').forEach(function (el) {
         el.style.display = (!labId || el.getAttribute('data-lab') === labId) ? '' : 'none';
     });
 }