Make EntityPicker child-aware

[iethree]

discussion

We want to make collection picker items aware of whether or not they have relevant children, so that, for example

• when picking a collection, you can tell if any given collection has more collection children
• when looking for a dashboard, we don't show you a dead-end path of collections that doesn't have any dashboards in it

This will likely be accomplished by providing some sort of below or here properties, telling us what entities exist below a given collection in the tree

This should be especially useful when we implement the picker for adding items to dashboards: we can hide any folders that don't have items that are addable to a dashboard.