Every way of deleting mail for good (JMAP, IMAP expunge, POP3, Trash
emptying, mailbox removal) notes the message's mailboxes and keywords while
archiving is on, fixing its deadline then; when its data is finally removed
it becomes an x:ArchivedItem record, written as upstream writes them, with
its copy held until the deadline. Retention is read at deletion time, so a
change applies at once. Restore puts a message back in the mailboxes it was
in (Trash only if that was all), with its keywords, and removes the record;
over quota it stays archived. x:ArchivedItem/get returns status and
accountId; query filters on type, archivedAt and text; set requests a
restore once or destroys; /changes is a fork addition. Expired items go in
the data purge. The shared account-access rule moves to jmap::inbuxa::access.
system_tests now calls undelete::test, and the archiving gate is gone.