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/proposals_controller.rb

@@ -92,19 +92,26 @@ class ProposalsController < ApplicationController
   private
 
     def proposal_params
+      params.require(:proposal).permit(allowed_params)
+    end
+
+    def allowed_params
       attributes = [:video_url, :responsible_name, :tag_list, :terms_of_service,
                     :geozone_id, :related_sdg_list,
                     image_attributes: image_attributes,
                     documents_attributes: document_attributes,
                     map_location_attributes: map_location_attributes]
       translations_attributes = translation_params(Proposal, except: :retired_explanation)
-      params.require(:proposal).permit(attributes, translations_attributes)
+
+      [*attributes, translations_attributes]
     end
 
     def retired_params
-      attributes = [:retired_reason]
-      translations_attributes = translation_params(Proposal, only: :retired_explanation)
-      params.require(:proposal).permit(attributes, translations_attributes)
+      params.require(:proposal).permit(allowed_retired_params)
+    end
+
+    def allowed_retired_params
+      [:retired_reason, translation_params(Proposal, only: :retired_explanation)]
     end
 
     def resource_model