api2/quarantine: add global sendlink api call

api2/quarantine: add global sendlink api call

this api call takes an email, checks it against the relay domains,
and prepares a custom quarantinelink for that email  and sends it there

this has to happen unauthenticated, since the idea is that the user
want to access the quarantine but has no current ticket (and no
old spam report with a ticket)

we rate limit the requests by allowing only a request per 5 seconds
(to prevent dos'ing the internal mail server) and only
one request per user/hour

this api call is disabled by default

if admins want even more ratelimiting, they can setup something
like fail2ban to block hosts hitting this api call often

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>