Make it easier to customize allowed parameters

When customizing CONSUL, one of the most common actions is adding a new
field to a form.

This requires modifying the permitted/allowed parameters. However, in
most cases, the method returning these parameters returned an instance
of `ActionController::Parameters`, so adding more parameters to it
wasn't easy.

So customizing the code required copying the method returning those
parameters and adding the new ones. For example:

```
def something_params
  params.require(:something).permit(
    :one_consul_attribute,
    :another_consul_attribute,
    :my_custom_attribute
  )
end
```

This meant that, if the `something_params` method changed in CONSUL, the
customization of this method had to be updated as well.

So we're extracting the logic returning the parameters to a method which
returns an array. Now this code can be customized without copying the
original method:

```
alias_method :consul_allowed_params, :allowed_params

def allowed_params
  consul_allowed_params + [:my_custom_attribute]
end
```

app/controllers/admin/banners_controller.rb

@@ -38,11 +38,13 @@ class Admin::BannersController < Admin::BaseController
   private
 
     def banner_params
-      attributes = [:target_url, :post_started_at, :post_ended_at,
-                    :background_color, :font_color,
-                    translation_params(Banner),
-                    web_section_ids: []]
-      params.require(:banner).permit(*attributes)
+      params.require(:banner).permit(allowed_params)
+    end
+
+    def allowed_params
+      [:target_url, :post_started_at, :post_ended_at, :background_color, :font_color,
+       translation_params(Banner),
+       web_section_ids: []]
     end
 
     def banner_styles