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/legislation/proposals_controller.rb

@@ -43,11 +43,17 @@ class Legislation::ProposalsController < Legislation::BaseController
   private
 
     def proposal_params
-      params.require(:legislation_proposal).permit(:legislation_process_id, :title,
-                    :summary, :description, :video_url, :tag_list,
-                    :terms_of_service, :geozone_id,
-                    image_attributes: image_attributes,
-                    documents_attributes: [:id, :title, :attachment, :cached_attachment, :user_id])
+      params.require(:legislation_proposal).permit(allowed_params)
+    end
+
+    def allowed_params
+      [
+        :legislation_process_id, :title,
+        :summary, :description, :video_url, :tag_list,
+        :terms_of_service, :geozone_id,
+        image_attributes: image_attributes,
+        documents_attributes: [:id, :title, :attachment, :cached_attachment, :user_id]
+      ]
     end
 
     def resource_model