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/users/registrations_controller.rb

@@ -60,9 +60,15 @@ class Users::RegistrationsController < Devise::RegistrationsController
 
     def sign_up_params
       params[:user].delete(:redeemable_code) if params[:user].present? && params[:user][:redeemable_code].blank?
-      params.require(:user).permit(:username, :email, :password,
-                                   :password_confirmation, :terms_of_service, :locale,
-                                   :redeemable_code)
+      params.require(:user).permit(allowed_params)
+    end
+
+    def allowed_params
+      [
+        :username, :email, :password,
+        :password_confirmation, :terms_of_service, :locale,
+        :redeemable_code
+      ]
     end
 
     def configure_permitted_parameters