Manage correctly results and stats for open-ended questions

Note that we are not including Poll::PartialResults for open-ended
questions resutls. The reason is that we do not contemplate the
possibility of there being open questions in booths. Manually
counting and introducing the votes in the system is not feasible.

config/locales/en/general.yml

@@ -639,6 +639,9 @@ en:
       results:
         title: "Questions"
         most_voted_answer: "Most voted answer: "
+        open_ended:
+          valid: Valid
+          blank: Blank
     poll_header:
       back_to_proposal: Back to proposal
   poll_questions:

config/locales/es/general.yml

@@ -639,6 +639,9 @@ es:
       results:
         title: "Preguntas"
         most_voted_answer: "Respuesta más votada: "
+        open_ended:
+          valid: Válidos
+          blank: En blanco
     poll_header:
       back_to_proposal: Volver a la propuesta
   poll_questions: