Expand options for adjacent_images endpoint

[manthey]

The adjacent_images endpoint is given an item ID and an optional virtual folder ID, from which it options the list of items in that folder, sorts them, and then finds the next and previous items from the specified one that are large_images.

This could be generalized where the folder ID could also be some ancestor folder along with an optional "flatten" flag. Further, the matching criteria for an item could be specified as a mongo query rather than explicitly just large_image. One motivation for this is to allow DIVE to leverage the same functionality, but it could also be eventually used for more sophisticated item browsing.

We could probably also allow a search within each folder or across the whole set.