add mailbox retention period to config

[krakazyabra]

Hello.
Unfortunately I'm not golang programmer, but I was able to fork and manually change retention period of mailbox from 24 hours to 30 min + cookie lifetime. Is it possible to put variable for this in env?
Such configuration can be useful for users, who are not ok with default 24h retention.

Thanks in advance ;-)

[haydenwoodhead]

I will happily review a pr that adds this as a config var and propagates it wherever it needs to go. However, it is not something I am interested in doing myself.