Reordering questions with drag'n'drop (#1433)

* Reordering questions with drag'n'drop

* Add permission check

* Test permissions for question reordering

* Handle malformed requests for question reordering

* Show first up arrow and last down arrow

* Provide page offset

* Revert "Provide page offset"

This reverts commit 8090bd573f.

* Reorder questions endpoint with pagination support

* Rudimentary test for reordering endpoint

* Make reordering questions atomic

* cache questions

* Properly support pagination for reorder_questions

* appease linter

* Fix test

src/pretix/control/templates/pretixcontrol/items/questions.html

@@ -39,9 +39,9 @@
                     <th class="action-col-2"></th>
                 </tr>
                 </thead>
-                <tbody>
+                <tbody data-dnd-url="{% url "control:event.items.questions.reorder" organizer=request.event.organizer.slug event=request.event.slug %}">
                 {% for q in questions %}
-                    <tr>
+                    <tr data-dnd-id="{{q.id}}">
                         <td><strong><a href="
                     {% url "control:event.items.questions.show" organizer=request.event.organizer.slug event=request.event.slug question=q.id %}">{{ q.question }}</a></strong>
                         </td>