Replace method for downloading current snapshot

Problem/Motivation

The current method has an endpoint that just gathers all the currently published datasets for a given theme and zips them up. This is not very efficient. They might be cached fine but probably aren't. We might need a way to delineate one current from another, like a hash of all the modified dates strung together.

This may not be needed, but creating an issue so it does not slip through the cracks.

Acceptance Criteria

Testing steps