comments: minimize sql queries.

Certain parts of comment rendering triggered sql queries that we didn't
really need to do. Rework things to avoid this.

* Preload comment creators in order to display commenter names with link_to_user.

* Preload comment votes in order to display "undo vote" links. Only preload
  votes for members since anonymous users can't vote and don't have "undo
  vote" links.

* Rework various conditionals to do the filtering in Ruby so that we
  avoid issuing any extra queries in sql.

* Avoid issuing any queries at all when the post doesn't have any
  comments (when last_commented_at is blank).

app/models/comment.rb

@@ -22,26 +22,6 @@ class Comment < ApplicationRecord
   )
 
   module SearchMethods
-    def recent
-      reorder("comments.id desc").limit(6)
-    end
-
-    def hidden(user)
-      if user.is_moderator?
-        where("(score < ? and is_sticky = false) or is_deleted = true", user.comment_threshold)
-      else
-        where("score < ? and is_sticky = false", user.comment_threshold)
-      end
-    end
-
-    def visible(user)
-      if user.is_moderator?
-        where("(score >= ? or is_sticky = true) and is_deleted = false", user.comment_threshold)
-      else
-        where("score >= ? or is_sticky = true", user.comment_threshold)
-      end
-    end
-
     def deleted
       where("comments.is_deleted = true")
     end
@@ -172,9 +152,16 @@ class Comment < ApplicationRecord
   end
 
   def voted_by?(user)
+    return false if user.is_anonymous?
     user.id.in?(votes.map(&:user_id))
   end
 
+  def visible_by?(user, show_thresholded: false, show_deleted: false)
+    return false if is_deleted? && !show_deleted && !user.is_moderator?
+    return false if score < user.comment_threshold && !is_sticky? && !show_thresholded
+    true
+  end
+
   def hidden_attributes
     super + [:body_index]
   end

app/models/post.rb

@@ -51,7 +51,7 @@ class Post < ApplicationRecord
   has_many :appeals, :class_name => "PostAppeal", :dependent => :destroy
   has_many :votes, :class_name => "PostVote", :dependent => :destroy
   has_many :notes, :dependent => :destroy
-  has_many :comments, -> {includes(:creator, :updater).order("comments.id")}, :dependent => :destroy
+  has_many :comments, -> {order("comments.id")}, :dependent => :destroy
   has_many :children, -> {order("posts.id")}, :class_name => "Post", :foreign_key => "parent_id"
   has_many :approvals, :class_name => "PostApproval", :dependent => :destroy
   has_many :disapprovals, :class_name => "PostDisapproval", :dependent => :destroy