Simplify methods to get API-public records

Just like we did to tags in commit 2e863fdc5.

app/models/comment.rb

@@ -37,12 +37,7 @@ class Comment < ApplicationRecord
   scope :sort_by_flags, -> { order(flags_count: :desc, updated_at: :desc) }
   scope :public_for_api, -> do
     not_valuations
-      .where(%{(comments.commentable_type = 'Debate' and comments.commentable_id in (?)) or
+      .where(commentable: [Debate.public_for_api, Proposal.public_for_api, Poll.public_for_api])
-            (comments.commentable_type = 'Proposal' and comments.commentable_id in (?)) or
-            (comments.commentable_type = 'Poll' and comments.commentable_id in (?))},
-          Debate.public_for_api.pluck(:id),
-          Proposal.public_for_api.pluck(:id),
-          Poll.public_for_api.pluck(:id))
   end
 
   scope :sort_by_most_voted, -> { order(confidence_score: :desc, created_at: :desc) }

app/models/proposal_notification.rb

@@ -10,7 +10,7 @@ class ProposalNotification < ApplicationRecord
   validates :proposal, presence: true
   validate :minimum_interval
 
-  scope :public_for_api,     -> { where(proposal_id: Proposal.public_for_api.pluck(:id)) }
+  scope :public_for_api,     -> { where(proposal: Proposal.public_for_api) }
   scope :sort_by_created_at, -> { reorder(created_at: :desc) }
   scope :sort_by_moderated,  -> { reorder(moderated: :desc) }
 

config/initializers/vote_extensions.rb

@@ -5,12 +5,7 @@ ActsAsVotable::Vote.class_eval do
   belongs_to :budget_investment, foreign_key: "votable_id", class_name: "Budget::Investment"
 
   scope :public_for_api, -> do
-    where(%{(votes.votable_type = 'Debate' and votes.votable_id in (?)) or
+    where(votable: [Debate.public_for_api, Proposal.public_for_api, Comment.public_for_api])
-            (votes.votable_type = 'Proposal' and votes.votable_id in (?)) or
-            (votes.votable_type = 'Comment' and votes.votable_id in (?))},
-          Debate.public_for_api.pluck(:id),
-          Proposal.public_for_api.pluck(:id),
-          Comment.public_for_api.pluck(:id))
   end
 
   def value