Don't allow to modify questions for started polls

Adding, modifiying, and/or deleting questions for an already started
poll is far away from being democratic and can lead to unwanted side
effects like missing votes in the results or stats.

So, from now on, only modifiying questions will be possible only if
the poll has not started yet.

app/controllers/admin/poll/questions_controller.rb

@@ -3,7 +3,8 @@ class Admin::Poll::QuestionsController < Admin::Poll::BaseController
   include Translatable
 
   load_and_authorize_resource :poll
-  load_and_authorize_resource :question, class: "Poll::Question"
+  load_resource class: "Poll::Question"
+  authorize_resource except: [:new, :index]
 
   def index
     @polls = Poll.not_budget
@@ -17,6 +18,8 @@ class Admin::Poll::QuestionsController < Admin::Poll::BaseController
     proposal = Proposal.find(params[:proposal_id]) if params[:proposal_id].present?
     @question.copy_attributes_from_proposal(proposal)
     @question.poll = @poll
+
+    authorize! :create, @question
   end
 
   def create