Implements single level nested checklist items in preparation for integration of checklist templates.
Checklist items can now be dragged into another for nesting. Order of checklist items is only updated and maintained upon nesting, while rearranging items within the list is not supported yet.
Dragging a checklist item into another nests it, and then the parent checklist item cannot be dragged. A nested checklist item can also be dragged into another top level checklist item, or back to the main checklist.
Once a top level checklist item contains no nested items, it becomes draggable. Deleting a top level checklist item deletes the nested items as well.
Nested checklist items can be minimized within the parent checklist item.

Basic CRUD of non-parent checklist items remain.
Ensured that CRUD functionality of parent checklist items is as expected - nested items are not affected, except when the parent is deleted.
Ensured proper draggable and droppable, and minimizing functionality depending on the state of each checklist item.
Also ensured that checklist item states persist after a refresh, for new items, parent items and child items.