Validate presence of 'poll_id' attribute on Poll::Question model

Fixes #1831

On branch aperez-validate-poll-question-is-selected
  Changes to be committed:
    modified:   app/models/poll/question.rb
    modified:   spec/models/poll/question_spec.rb
    modified:   spec/features/admin/poll/polls_spec.rb

spec/models/poll/question_spec.rb

@@ -9,6 +9,20 @@ RSpec.describe Poll::Question, type: :model do
     end
   end
 
+  describe "#poll_question_id" do
+    it "should be invalid if a poll is not selected" do
+      q = create(:poll_question)
+      q.poll_id = nil
+      expect(q).to_not be_valid
+    end
+
+    it "should be valid if a poll is selected" do
+      q = create(:poll_question)
+      q.poll_id = 1
+      expect(q).to be_valid
+    end
+  end
+
   describe "#copy_attributes_from_proposal" do
     it "copies the attributes from the proposal" do
       create_list(:geozone, 3)