FEATURE: add "email in-reply-to user option"

We no longer include previous replies as "context", instead
we include and excerpt of the post being replied to at the bottom
of notifications, this information was previously missing.

Users may opt in to emailing previous replies if they wish
or opt out of "in-reply-to" which makes sense in some email clients that
are smarter about displaying a tree of replies.

db/migrate/20160225095306_add_email_in_reply_to_to_user_options.rb

@@ -0,0 +1,11 @@
+class AddEmailInReplyToToUserOptions < ActiveRecord::Migration
+  def up
+    add_column :user_options, :email_in_reply_to, :boolean, null: false, default: true
+    change_column :user_options, :email_previous_replies, :integer, default: 2, null: false
+    execute 'UPDATE user_options SET email_previous_replies = 2'
+  end
+
+  def down
+    remove_column :user_options, :email_in_reply_to
+  end
+end