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.
1. I think we could make a custom route for this (e.g. ```/registries/:registry_id/repositories/:id```), that maps to this same action. I couldn't decide if it was worth it or not... It may actually complicate things, b/c I'm making the registry_id optional
2. ```Registry.first```, maybe we should actually look the seeded registry up by name (Docker Hub) or whatever, I must admit Registry.first is a pretty weak.