Move the sessions helper to a concern

Helpers should be used exclusively on the views as a way to format short pieces of data. The responsibility of this helper is way big and it belongs to a controller. This is done via an `ActiveSupport::Concern` (http://api.rubyonrails.org/classes/ActiveSupport/Concern.html).

If you want to expose some of its methods to a view, you can use the convenient `helper_method` macro.