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/account_controller.rb

@@ -22,14 +22,17 @@ class AccountController < ApplicationController
     end
 
     def account_params
-      attributes = if @account.organization?
-                     [:phone_number, :email_on_comment, :email_on_comment_reply, :newsletter,
-                      organization_attributes: [:name, :responsible_name]]
-                   else
-                     [:username, :public_activity, :public_interests, :email_on_comment,
-                      :email_on_comment_reply, :email_on_direct_message, :email_digest, :newsletter,
-                      :official_position_badge, :recommended_debates, :recommended_proposals]
-                   end
-      params.require(:account).permit(*attributes)
+      params.require(:account).permit(allowed_params)
+    end
+
+    def allowed_params
+      if @account.organization?
+        [:phone_number, :email_on_comment, :email_on_comment_reply, :newsletter,
+         organization_attributes: [:name, :responsible_name]]
+      else
+        [:username, :public_activity, :public_interests, :email_on_comment,
+         :email_on_comment_reply, :email_on_direct_message, :email_digest, :newsletter,
+         :official_position_badge, :recommended_debates, :recommended_proposals]
+      end
     end
 end