Implement the bundles-containing API endpoint.

I think I'd lose the entitiesFilterer type - I personally feel that func(*mongodoc.Entity) bool is
more descriptive than the entitiesFilterer name.

It would also be fairly trivial to make this just update the slice in place, a fairly
standard idiom:

    j := 0
    for i := range entities {
        entity := &entities[i]
        if predicate(entity) {
            entities[j] = *entity
            j++
        }
     }
     return entities[0:j]