Feature/search across registries

This enables searching across all registered registries, but we only seed docker hub, so this *shouldn't* change anything for end users today. This may break the tags stuff if used with private registry, which I am tackling next.
Is there a way we could seed the Docker Hub registry into the DB so that it was always `id: 0`? And then default the `registry_id` param to 0 if it isn't provided?

That feels (only slightly) less janky than `.first`