Avoid sending sort events twice

SortableJS sends two events if you drag an element from one
nesteable elements container to another. Leading to incrementing
the target lists positions to high.

Since acts_as_list maintains both lists correctly we can simply
suppress the second event on the source list.

app/javascript/alchemy_admin/sortable_elements.js

@@ -29,12 +29,17 @@ function onSort(event) {
     params.parent_element_id = parentElement.dataset.elementId
   }
 
-  post(Alchemy.routes.order_admin_elements_path, params).then((response) => {
-    const data = response.data
-    Alchemy.growl(data.message)
-    Alchemy.PreviewWindow.refresh()
-    item.updateTitle(data.preview_text)
-  })
+  // Only send the request if the item was moved to a different container
+  // or sorted in the same list. Not on the old list in order to avoid incrementing
+  // the position of the other elements.
+  if (event.target === event.to) {
+    post(Alchemy.routes.order_admin_elements_path, params).then((response) => {
+      const data = response.data
+      Alchemy.growl(data.message)
+      Alchemy.PreviewWindow.refresh()
+      item.updateTitle(data.preview_text)
+    })
+  }
 }
 
 function onEnd() {