Use AR relations when merging comments

Using arrays made it difficult to order by more than one field (like the
`most_voted` scope does), and so we were ordering by `confidence_score`
and ignoring the `created_at` column.

Using an AR relation makes it easy to reuse the existing `most_voted`
scope.

This change has one side effect: now comments with equal votes are
ordered in descending order instead of having no specific order. That
means flaky specs which failed sometimes because they assumed comments
were ordered by date are now always green.

I've also re-added the `oldest` scope removed in 792b15b thinking it was
removed because using it with arrays was too hard.

lib/merged_comment_tree.rb

@@ -1,24 +1,13 @@
 class MergedCommentTree < CommentTree
-  attr_reader :commentables, :array_order
+  attr_reader :commentables
 
   def initialize(commentables, page, order = "confidence_score")
     @commentables = commentables
     super(commentables.first, page, order)
-    @array_order = set_array_order(order)
   end
 
-  def root_comments
-    Kaminari.paginate_array(commentables.flatten.map(&:comments).map(&:roots).flatten.sort_by {|a| a[array_order]}.reverse).
-    page(page).per(ROOT_COMMENTS_PER_PAGE)
-  end
-
-  def set_array_order(order)
-    case order
-    when "newest"
-      "created_at"
-    else
-      "confidence_score"
-    end
+  def base_comments
+    Comment.where(commentable: commentables.flatten)
   end
 
 end