chore: General fixes and clean up (#1169)

app/models/application_record.rb

@@ -1,7 +1,11 @@
 class ApplicationRecord < ActiveRecord::Base
+  include Events::Types
   self.abstract_class = true
+
+  # the models that exposed in email templates through liquid
   DROPPABLES = %w[Account Channel Conversation Inbox User].freeze
 
+  # ModelDrop class should exist in app/drops
   def to_drop
     return unless DROPPABLES.include?(self.class.name)