Chatmail: Automatic data/account deletion

[s0ph0s-dog]

This is part of a series of issues tracking chatmail support in Mox.

In order to minimize the amount of data retained on the mail server, chatmail servers impose the following mandatory data deletion limits:

• All mails are deleted 30 days after receipt
• All chatmail accounts are deleted 90 days after the most recent log in

These limits should be configurable by the server administrator.

Open question: Should the data retention limits be configurable per-user? (Up to but not exceeding the server-wide limit)

• Add goroutine which waits in the background and wakes itself once per day.
• Make that goroutine delete all emails in all user accounts which were received more than 30 days ago.
• Make that goroutine delete all user accounts marked as chatmail-enabled and which have a last log-in date of more than 90 days ago.
• Add configuration options to control the data retention limits in the domains.conf.