Mail a different text when there are no links for a week, fixes #7

In #7 we discussed what should happen when there are no links for a week. The current behaviour was that a mail with an awkward content for no links would be sent. The possibilities discussed were: Don't send a mail at all or send some other kind of mail. The first option would confuse some users and make them forget potter at best, so we opted for the second option, sending a different mail. This commit adds a more descriptive text if there are no links.