Archive old process mailboxes

Some time after a process is terminated, its mailbox can be archived. This could go as such:

• a housekeeping job detects malboxes to archive, and compresses them
• the archive-process-email script checks if a compressed version of the target mailbox exists, and if so, throws away the mail

While one edits archive-process-email, one can also remove all code related to legacy/old-style processes.

The process.views.DisplayMailArchive view needs to be updated to deal with compressed mailboxes.