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/concerns/admin/budget_groups_actions.rb

@@ -54,7 +54,12 @@ module Admin::BudgetGroupsActions
     end
 
     def budget_group_params
+      params.require(:budget_group).permit(allowed_params)
+    end
+
+    def allowed_params
       valid_attributes = [:max_votable_headings]
-      params.require(:budget_group).permit(*valid_attributes, translation_params(Budget::Group))
+
+      [*valid_attributes, translation_params(Budget::Group)]
     end
 end

app/controllers/concerns/admin/budget_headings_actions.rb

@@ -55,7 +55,12 @@ module Admin::BudgetHeadingsActions
     end
 
     def budget_heading_params
+      params.require(:budget_heading).permit(allowed_params)
+    end
+
+    def allowed_params
       valid_attributes = [:price, :population, :allow_custom_content, :latitude, :longitude, :max_ballot_lines]
-      params.require(:budget_heading).permit(*valid_attributes, translation_params(Budget::Heading))
+
+      [*valid_attributes, translation_params(Budget::Heading)]
     end
 end

app/controllers/concerns/admin/budget_phases_actions.rb

@@ -40,8 +40,13 @@ module Admin::BudgetPhasesActions
     end
 
     def budget_phase_params
+      params.require(:budget_phase).permit(allowed_params)
+    end
+
+    def allowed_params
       valid_attributes = [:starts_at, :ends_at, :enabled,
                           image_attributes: image_attributes]
-      params.require(:budget_phase).permit(*valid_attributes, translation_params(Budget::Phase))
+
+      [*valid_attributes, translation_params(Budget::Phase)]
     end
 end

app/controllers/concerns/admin/widget/cards_actions.rb

@@ -40,11 +40,15 @@ module Admin::Widget::CardsActions
   private
 
     def card_params
-      params.require(:widget_card).permit(
+      params.require(:widget_card).permit(allowed_params)
+    end
+
+    def allowed_params
+      [
         :link_url, :button_text, :button_url, :alignment, :header, :columns,
         translation_params(Widget::Card),
         image_attributes: image_attributes
-      )
+      ]
     end
 
     def header_card?