Fix filtering by line item in admin

[nirebu]

Summary

Fixes #274

This PR fixes the above issue by changing how the admin scopes subscriptions tied to a particular variant, instead of having to hunt down the specific line item when filtering them.

This also restructures the query in the view to avoid N+1s. On a store with 100K subscriptions the query and relative map take ~30ms.

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[chrean]

Thank you @nirebu and @stefano-sarioli (and @kennyadsl )!