feat: Show last modified by user on document viewer

- Display updater name next to update time
- Fall back to creator if no updater is set
- Add modified_by translation for all 8 languages

src/lang/zh-TW/messages.php

@@ -22,6 +22,7 @@
         'save' => '儲存',
         'cancel' => '取消',
         'created_by' => '建立者',
+        'modified_by' => '修改者:',
         'updated' => '更新時間',
         'path' => '路徑',
         'last_modified' => '最後修改',