Add option to not directly remove deleted mailboxes

When a mail account is deleted via the interface it's directly removed from the filesystem.

While we have a very nice undo action in the datalog history that does not bring back the data.

A regular backup will probably have a gap between when it finished and when the mailbox is deleted. In which changed can occur which we are not able to recover.

And in some cases it might be a compliance issue to purge mailboxes.

I suggest we add an option to delay the deletion.

One way could be to rename the mail folder to e.g. exmaple.com/mailuser-20210318222513. Renaming will not hold-up the task queue with large mailboxes.

A cronjob could then process these further. There are multiple ways to do that and I expect opinions to vary on that.

• remove after x time
• compress into a tar
• sent to an archive location
• leave it and have your own cleanup task
• leave it for manual cleanup

As a bonus we could (under certain conditions) move it back when an undo action is performed.

I've started with code to rename mail_user and mail_domain directories. See the linked MR.

Thoughts?