mod_firewall: SUBSCRIBED: Flip subscription check to match documentation
The documentation claims that this condition checks whether the recipient is
subscribed to the sender.
However, it was using the wrong method, and actually checking whether the
sender was subscribed to the recipient.
A quick poll of folk suggested that the documentation's approach is the right
one, so this should fix the code to match the documentation.
This should also fix the bundled anti-spam rules from blocking presence from
JIDs that you subscribe do (but don't have a mutual subscription with).
