Add a script to periodically remove old IP addresses from the users
database.

The login IP addresses are stored for spam protection and to prevent
from abuse. It is quite unlikely that we ever need the IP address of a
user whose last login is more than a week old. It makes sense to remove
such IP addresses to protect our users' privacy.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>