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 8090bd573f851a74cca442f4651c111b8750948d.

* 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/static/pretixcontrol/js/ui/dragndroplist.js

@@ -0,0 +1,45 @@
+/*global $, Sortable*/
+$(function () {
+    $("[data-dnd-url]").each(function(){
+        var container = $(this),
+            url = container.data("dnd-url"),
+            up = container.find("a:has(.fa-arrow-up)"),
+            handle = $('<span class="btn btn-default btn-sm dnd-sort-handle"><i class="fa fa-arrows"></i></span>');
+
+        function hideArrows(container){
+            var up = container.find("a:has(.fa-arrow-up)"),
+                firstUp = up.first(),
+                down = container.find("a:has(.fa-arrow-down)"),
+                lastDown = down.last();
+            up.not(firstUp).css("display","none");
+            down.not(lastDown).css("display","none");
+            firstUp.css("display","inline-block");
+            lastDown.css("display","inline-block");
+        }
+        up.after(handle);
+        hideArrows(container);
+
+        Sortable.create(container.get(0), {
+            handle: ".dnd-sort-handle",
+            onSort: function(evt){
+                var container = $(evt.to),
+                    ids = container.find("[data-dnd-id]").toArray().map(e => e.dataset.dndId);
+
+                hideArrows(container);
+
+                $.ajax(
+                    {
+                        'type': 'POST',
+                        'url': url,
+                        'headers': {'X-CSRFToken': $("input[name=csrfmiddlewaretoken]").val()},
+                        'data': JSON.stringify({
+                            ids: ids
+                        }),
+                        'contentType': "application/json",
+                        'timeout': 30000
+                    }
+                );
+            }
+        });
+    });
+});