Refactor resolver

[pablobm]

A few issues currently open are related to how the resolver works. I have tagged them with resolver :

• Errors for STI models (undefined method for paths) #703
• Namespaced models not working #1291
• ActiveStorage "media library" / dashboards for blobs and attachments #1560
• Using two instances of Administrate with their own dashboard #1764

Problems including dealing with STI models, namespaced models, and the possibility of running separate admin interfaces.

I think a lot of this could be fixed by allowing users to configure the parameters that the resolver receives. So for example declare that a given set of controllers are going to use dashboards under a specific namespace.

Some of this could also be made easier if the resolver was a bit less clever. As I mention at #405 (comment), it tries to do a lot from very little, while it could receive more information than it does and at the same time be more configurable.