Update lots of things including mailers

* This is supposed to be all mailers but I've updated a lot of
  other things related to the app
* Update how the due dates work... set up a method that marks all
  rent instances for a specific property and billing period as paid
  if one user pays for that billing period
* Group by billing period for properties showing all rent periods
  that rent has been either created or paid and who is responsible for
  that current billing periods rent
* created a mailer after a user has paid their rent
* Set mailer up as a background mailer