Remove all paths on Element deletion

It was not possible to remove Element instances due to the ElementPaths. The array of UUIDs describing the paths technically is an array of foreign keys, but is only defined as an array of UUIDs. This adds a Django signal to automatically remove those paths by calling remove_child on all the direct parents of an Element instance, and sets the element foreign key deletion mode to cascade. This allows to quickly remove an entire corpus using the Django admin for example, which is very useful during development.