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/templates/admin/labs.html

@@ -19,13 +19,13 @@
 
     <div class="card card-flush">
         <table id="labs-table" class="tablekit">
-            <thead><tr><th class="th-sortable is-asc" title="Номер — по возрастанию">№</th><th>Название</th><th>Вариантов</th><th>Статус</th><th data-nosort></th></tr></thead>
+            <thead><tr><th class="th-sortable is-asc" title="Номер — по возрастанию">№</th><th>Название</th><th>Вариантов</th><th data-filter>Статус</th><th data-nosort></th></tr></thead>
             <tbody>
             <tr th:each="lab : ${labs}">
                 <td th:text="${lab.number}">1</td>
                 <td><a th:href="@{/admin/labs/{id}(id=${lab.id})}" th:text="${lab.title}">—</a></td>
                 <td th:text="${lab.variantCount}">0</td>
-                <td>
+                <td th:attr="data-filter-value=${lab.status}">
                     <form class="inline-form" th:action="@{/admin/labs/{id}/toggle-status(id=${lab.id})}" method="post">
                         <input type="hidden" name="_csrf" th:value="${csrfToken}">
                         <button class="badge badge-btn"