Fix popup menus

We need to use Popper 1.12.6 until there's a remedy to
https://github.com/FezVrasta/popper.js/issues/532

skins/elastic/templates/includes/footer.html

@@ -7,7 +7,8 @@
 
 <roundcube:object name="message" id="messagestack" />
 
-<script src="/bootstrap/bootstrap.bundle.min.js"></script>
+<script src="/bootstrap/popper.min.js"></script>
+<script src="/bootstrap/bootstrap.min.js"></script>
 <script src="/ui.js"></script>
 
 </body>