Implement the bundles-containing API endpoint.

I'm finding this logic a little hard to understand.
How about removing one level of abstraction and just implementing
the predicate function inline here?

    match := func(e *mongodoc.Entity) bool {
        for _, cid := range entity.BundleCharms {
            if cid.Name == id.Name &&
                cid.User == id.User &&
                (anySeries || cid.Series == id.Series) &&
                (anyRevision || cid.Revision == id.Revision) {
                return true
            }
        }
        return false
    }
    entities = filterEntities(entities, match)