Add Administrate::Punditize methods as module methods (thoughtbot#2403)

Instead of adding them via `included do`.

If they are added via "included do" it makes it hard to override the method
in an app's controller.

Example:

```ruby
module Admin
  class ApplicationController < Administrate::ApplicationController
    include Administrate::Punditize

    def scoped_resource
      super.where(archived: false)
    end
  end
end
```

That example will skip pundit completely, because the `def scoped_resource` from
`Administrate::Punditize` was added via `included do`, which means it will
behave as if we had defined the method twice in `Admin ::ApplicationController`,
which will result in the first definition from `Administrate::Punditize` being
ignored. And "super" will refer to the no-op definition provided in the base
class `Administrate::ApplicationController`.

This seems unexpected to me, and makes it hard to add functionality that layers
on to pof what `Administrate::Punditize` does.

However, if we defined `def scoped_resource` as a module method in
`Administrate::Punditize` then `super` in `Admin::ApplicationController` will
refer to the method defined in `Administrate::Punditize`.

app/controllers/concerns/administrate/punditize.rb

@@ -9,31 +9,27 @@ module Punditize
         include Pundit
       end
 
-      included do
-        private
-
-        def policy_namespace
-          []
-        end
+      private
 
-        def scoped_resource
-          namespaced_scope = policy_namespace + [super]
-          policy_scope!(pundit_user, namespaced_scope)
-        end
+      def policy_namespace
+        []
+      end
 
-        def authorize_resource(resource)
-          namespaced_resource = policy_namespace + [resource]
-          authorize namespaced_resource
-        end
+      def scoped_resource
+        namespaced_scope = policy_namespace + [super]
+        policy_scope!(pundit_user, namespaced_scope)
+      end
 
-        def authorized_action?(resource, action)
-          namespaced_resource = policy_namespace + [resource]
-          policy = Pundit.policy!(pundit_user, namespaced_resource)
-          policy.send("#{action}?".to_sym)
-        end
+      def authorize_resource(resource)
+        namespaced_resource = policy_namespace + [resource]
+        authorize namespaced_resource
       end
 
-      private
+      def authorized_action?(resource, action)
+        namespaced_resource = policy_namespace + [resource]
+        policy = Pundit.policy!(pundit_user, namespaced_resource)
+        policy.send("#{action}?".to_sym)
+      end
 
       def policy_scope!(user, scope)
         policy_scope_class = Pundit::PolicyFinder.new(scope).scope!