Don't move task to calendar when dropped outside

At the moment when you drop a dragged task outside of a calendar, it is inserted at the last valid position. This might be unexpected to the user and effectively prevents canceling a drag and drop operation.

With Sortable v1.10.0 and the OnSpill plugin https://github.com/SortableJS/Sortable/tree/master/plugins/OnSpill this can be improved once the new Sortable.js version is released.