This API endpoint is intended for usage with the entire `solo_scores`
machinery and ID schema, rather than the legacy `*_scores_high` ID
schema. It also supports automagically falling back to downloading
legacy replays if a stable-imported score is requested for download
(internally this happens via `legacy_score_id` in the `data` json).
This change will allow replays to be downloaded, but it will still not
yield 100% correct behaviour, as there is further work to be done in
that respect. The download tracker is expecting score hashes to arrive
from web to verify the integrity of the incoming download, but the API
does not expose such a facility right now; we will have to decide as to
whether we want to add one web-side, or whether we want to disable the
checking client-side.