add back sort button disabled state management

src/pretix/static/pretixcontrol/js/ui/dragndroplist.js

@@ -1,7 +1,27 @@
 /*global $, Sortable*/
 $(function () {
+    const allContainers = $("[data-dnd-url]");
+    function updateAllSortButtonStates() {
+        allContainers.each(function() { updateSortButtonState($(this)); });
+    }
+    function updateSortButtonState(container) {
+        var disabledUp = container.find(".sortable-up:disabled"),
+            firstUp = container.find(">tr[data-dnd-id] .sortable-up").first();
+        if (disabledUp.length && disabledUp.get(0) !== firstUp.get(0)) {
+            disabledUp.prop("disabled", false);
+            firstUp.prop("disabled", true);
+        }
+
+        var disabledDown = container.find(".sortable-down:disabled"),
+            lastDown = container.find(">tr[data-dnd-id] .sortable-down").last();
+        if (disabledDown.length && disabledDown.get(0) !== lastDown.get(0)) {
+            disabledDown.prop("disabled", false);
+            lastDown.prop("disabled", true);
+        }
+    }
+
     let didSort = false, lastClick = 0;
-    $("[data-dnd-url]").each(function(){
+    allContainers.each(function(){
         const container = $(this),
             url = container.data("dnd-url"),
             handle = $('<span class="btn btn-default btn-sm dnd-sort-handle"><i class="fa fa-arrows"></i></span>');
@@ -19,6 +39,7 @@ $(function () {
         function maybeShowSortButtons() {
             if (Date.now() - lastClick < 3000) {
                 $("[data-dnd-url] .sortable-up, [data-dnd-url] .sortable-down").removeClass("sr-only");
+                updateAllSortButtonStates();
             }
             lastClick = Date.now();
         }