Only allow one appeal at a time

[dqwiki]

Currently more than one appeal can be filed by the same user while one is open. We need to close this gap so we don't overwhelm admins reviewing.

[Salvidrim]

Appeals filed from the same IP are automatically listed and should be possible to disallow; appeals made from different IPs aren't and would need to be matched by some other criteria (e-mail address, probably)