Create an ApplicationRecord for models to inherit from

This is the default for Rails 5+, and also paves the way for
multiple database support.

app/models/issue_comment.rb

@@ -20,7 +20,7 @@
 #  issue_comments_user_id_fkey   (user_id => users.id)
 #
 
-class IssueComment < ActiveRecord::Base
+class IssueComment < ApplicationRecord
   belongs_to :issue
   belongs_to :user