Notifications sent for destroyed links can be noisy

[ichthyos]

If someone destroys a link, we currently send alerts for whatever the first url in the email is, which might not be the portal that the player is attacking.

We should probably modify mail.py to alert for up to two urls. At the same time, perhaps we could change the alert text in the multiple url case to something like "Alert! %s (%s) may be under attack!" What do you think?