Disallow to modify the start date for an already started poll

We need to update a couple of tests because a poll is created in the
tests with a timestamp that includes nanoseconds and in the form to edit
the time of the poll the nanoseconds are not sent, meaning it was
detected as a change.

config/locales/es/general.yml

@@ -138,6 +138,8 @@ es:
       user_not_found: Usuario no encontrado
       invalid_date_range: "El rango de fechas no es válido"
       past_date: "No puede ser una fecha pasada"
+      cannot_change_date:
+        poll_started: "No puede ser cambiada si la votación ha comenzado"
   form:
     accept_terms: Acepto la %{policy} y las %{conditions}
     accept_terms_title: Acepto la Política de privacidad y las Condiciones de uso