Provide an API endpoint for legacy mobile clients to retrieve final disposition of finished splits

[jmileham]

Depends on #74.

An unauthenticated endpoint to ask for the decided variant of any finished split.

Because there was not always a decision field on splits, this should probably error if a requested split exists and is finished but has no decision to give developers an opportunity to backfill that split's decision with the correct value.

The assumption is that the mobile clients will permanently cache the results of this endpoint to disk, so we only get one shot to provide the correct final decision for a split before that client locks in its behavior indefinitely.

This endpoint would 404 for non-finished splits, or splits that aren't defined at all.