Drag & drop hierarchies in sidebar

[joepio]

• [x] see children of children in the sidebar
• [x] collapse / open parents in the sidebar
• [ ] change the parent of a resource by dropping a resource in the sidebar. Ideally users can drag from various contexts (e.g. using the editableTitle Component)
• [ ] Sort items and change their ordering using subResources property. See how this is done in handleSortEnd in DocumentPage.tsx.

Probably best to use dnd-kit, as we already use this in Documents for changing sort order of Elements.

@adileo also wants this, might be willing to work on this