/api/collection/:id/items gets `here` and `below` (#40203)

* /api/collection/:id/items gets `here` and `below`

During postprocessing of `collection` children retrieved by
`/api/collection/:id/items`, annotate each of the children with `below`
and `here` keys representing the presence of questions, models, or
collections at this level of the hierarchy or below it.

Co-authored-by: Noah Moss <32746338+noahmoss@users.noreply.github.com>