Track refunds on donations

So we can distinguish between "never paid" and "unpaid".
Make url_helpers available to all mailer tests

This is a common thing since you want to test links in emails.
Autocancel requests flagged too long

* Now there are two reasons for autocancel (open-too-long and flagged-
  too-long)
* If a flagged request is autocanceled, all relevant parties get a
  notification about it
* The donation is canceled too and is refunded
* The student can reopen the request but the old donation is removed
Remove old mail example
Display user balance if any

On home page and on donate page.