comments: add fix script to remove duplicate votes.

There are about 100 duplicate comment votes. This is because there
wasn't a uniqueness constraint in the database to prevent duplicate
votes. This adds a script to remove duplicate votes so that a constraint
can be added later.

script/fixes/074_delete_duplicate_comment_votes.rb

@@ -0,0 +1,14 @@
+#!/usr/bin/env ruby
+
+require_relative "../../config/environment"
+
+CommentVote.transaction do
+  CommentVote.group(:comment_id, :user_id).having("count(*) > 1").count.each do |(comment_id, user_id), count|
+    votes = CommentVote.where(comment_id: comment_id, user_id: user_id).order(:id)
+
+    # Remove all but the first duplicate vote.
+    dupe_votes = votes.drop(1)
+    dupe_votes.each { p _1 }
+    dupe_votes.each(&:destroy)
+  end
+end