Add option_id to partial results and unique index

Similar to what we did in PR "Avoid duplicate records in poll answers" 5539,
specifically in commit 503369166, we want to stop relying on the plain text
"answer" and start using "option_id" to avoid issues with counts across
translations and to add consistency to the poll_partial_results table.

Note that we also moved the `possible_answers` method from Poll::Question to
Poll::Question::Option, since the list of valid answers really comes from the
options of a question and not from the question itself. Tests were updated
to validate answers against the translations of the assigned option.

Additionally, we renamed lambda parameters in validations to improve clarity.

app/models/poll/question.rb

@@ -49,10 +49,6 @@ class Poll::Question < ApplicationRecord
     question_options.max_by(&:total_votes)&.id
   end
 
-  def possible_answers
-    question_options.joins(:translations).pluck(:title)
-  end
-
   def options_with_read_more?
     options_with_read_more.any?
   end