Add support for open-ended questions in admin section

Introduce a new "open" votation type for poll questions in the admin
interface. This type allows open answers provided by the user.

app/models/poll/question.rb

@@ -27,7 +27,7 @@ class Poll::Question < ApplicationRecord
   accepts_nested_attributes_for :question_options, reject_if: :all_blank, allow_destroy: true
   accepts_nested_attributes_for :votation_type
 
-  delegate :multiple?, :vote_type, to: :votation_type, allow_nil: true
+  delegate :multiple?, :open?, :vote_type, to: :votation_type, allow_nil: true
 
   scope :sort_for_list, -> { order(Arel.sql("poll_questions.proposal_id IS NULL"), :created_at) }
   scope :for_render,    -> { includes(:author, :proposal) }