Periodic user mailer

We wanted to make the stuff-to-do plugin a bit more active, so we created a mailer that we can schedule to send periodically via the new `rake redmine:stuff_to_do:send_periodic_mailer` rake task. When invoked, it will send a notification email to each user who has stuff to do (it will skip over users who don't have anything set in their `doing_stuff` or `recommended` stuff-to-do queues.