Add correct hyperlinks on nested models in collections

Created by: paulRbr

Context

E.g. In the following routes, Order model can only be shown (or edited) when it's accessed via it's nested /customers/:customer_id/orders route. If we imagine order ids to be dependent of customers (Order#1 for Customer#1 and Order#1 for Customer#2):

Rails.application.routes.draw do
  namespace(:admin) do
    resources :customers do
      resources :orders, only: [:show, :edit]
    end
  end
end

Until now, administrate would not link the associated models in collections when their actions can only work correctly with the existence of a parent resource (by defining a Admin::OrdersController#find_resource method to find the nested resource depending on URL params).

Suggestion

This PR tries to allow models which are nested within a parent related model to be accessed via a nested URL.

I'm not 100% happy with the current codebase (it has lots of duplication) but I wanted to share the main idea which works for us @bump-sh.

Please let me know if this feature could see life in Administrate and how I can make this suggestion better? Thanks!