Normalizes copy product navigation_helper#link_to_clone

I had to use `link_to_clone` for another resource, not a product. Since the `clone_admin_product_url` was fixed in the method, I had to duplicate it.

I would like to suggest to normalize this method to work similar to `:new_object_url, :edit_object_url, :object_url, :collection_url`.

I added the `helper_method` directly in the ProductController instead of ResourceController, because I don't know how frequently it will be used by others resources. If it makes sense to add it in the ResourceController I can change the PR.

**Usage:**

```ruby
module Spree::Admin
  class MyResourceController  < ResourceController
    helper_method :clone_object_url

    protected

    def clone_object_url resource
      clone_admin_my_resource_url resource
    end
  end
end
```
--

```erb
<%= link_to_clone my_resource, no_text: true %>
```