Learn UI: prev/next navigation, sidebar outline, editing on every page; merge import; defense date

- article page: ← / → navigation across the whole material tree (including
  between sections), sticky sidebar outline with the current article highlighted
- editing on chapter, section and article pages: rename, publish/hide, delete,
  create section/article, move articles while deleting a section; forms return
  to the page they were opened from (validated internal `back` parameter)
- article page opens the edit dialog directly via ?edit=1 (used by "Править"
  links on section/chapter pages)
- import: optional sync flag — with it stale sections/articles of archived
  chapters are deleted (previous behaviour), without it import only adds and
  updates, so material edited in the UI is not overwritten
- student area: lab page and lab cards now show the defense date next to the
  submission deadline
- fix SpEL crash on the student lab page (`and` with nullable strings)

web/src/main/resources/templates/learn/index.html

@@ -198,6 +198,14 @@
                 </p>
                 <div><label class="label" for="file">Архив</label>
                     <input class="input" id="file" name="file" type="file" accept=".zip" required></div>
+                <label class="tone-label">
+                    <input type="checkbox" name="sync" value="true">
+                    Удалять разделы и статьи, которых нет в архиве (синхронизация)
+                </label>
+                <p class="hint">
+                    Без этой галочки импорт <b>только добавляет и обновляет</b> материалы из архива —
+                    статьи, отредактированные прямо в приложении, не затрагиваются.
+                </p>
                 <button class="btn btn-primary" type="submit">Импортировать</button>
             </form>
         </div>