Fixed Drag and drop between checklists closes the card sometimes on Firefox.

xet7 thanks mfilser about [similar fix of appendTo parent](https://github.com/wekan/wekan/pull/3342)
that did work here too to fix this.

Thanks to mfilser and xet7 !

Fixes #2280

client/components/cards/checklists.js

@@ -6,7 +6,7 @@ function initSorting(items) {
     helper: 'clone',
     items: '.js-checklist-item:not(.placeholder)',
     connectWith: '.js-checklist-items',
-    appendTo: '.board-canvas',
+    appendTo: 'parent',
     distance: 7,
     placeholder: 'checklist-item placeholder',
     scroll: false,