FEATURE: allow users to control how many previous replies they get

- always means we always send previous replies with every email
- never means we do not
- "unless previously sent" ... is the default, in which we only email you each reply once

The default_email_previous_replies site setting can control this toggle

db/migrate/20160225050320_add_email_previous_replies_to_user_options.rb

@@ -0,0 +1,5 @@
+class AddEmailPreviousRepliesToUserOptions < ActiveRecord::Migration
+  def change
+    add_column :user_options, :email_previous_replies, :integer, null: false, default: 1
+  end
+end