use fetchmail for donor import

The former approach to export a maildir and iterate over it with a script broke when the mail server and the web server got on their own hosts. This will use IMAP IDLE to check for new mails and pass them instantly to the djange manage.py script without storing the mail locally.

Only apply with https://github.com/archlinux/archweb/pull/325

When applied, use fetchmail parameters keep and fetchall for the first run to ensure no error screws up the process.