Sending Email Enhancements (bug: LP#1195856)

This patch will make two changes.

1. Remove the requirement to have a limit when sending outbound email
   (you can set it to false for no limit).

2. Add a new flag that will merge outbound emails to different
   recipients, that have the same contents, into one email with multiple
   recipients. These changes are centralized into the sending cron job
   procedure so they require no DDL changes or other changes to core
   parts of the application.