enable an @ here ping for instance down alerts

This should cause an InstanceDown alert to ping @here in the GitHost slack channel.

changed title from enable an @here ping for instance down alerts to enable an @ here ping for instance down alerts

Sorry @here, I didn't realize that it would parse the @ name in the title.

@lbot @cpallares I'm going to merge this. If it gets out of hand, we can always disable it later.

merged