Show a loading message before first page of history is loaded.

realms/templates/wiki/history.html

@@ -14,6 +14,9 @@
       <th>Date</th>
     </tr>
     </thead>
+    <tbody>
+    <tr><td colspan="3" style="text-align: center">Loading file history...</td></tr>
+    </tbody>
   </table>
   <p>
     <a class="btn btn-default btn-sm compare-revisions">Compare Revisions</a>