Manually expunge old `mail_room` messages

As per @cmiskell 's comment, we should perform this before merging the mail_room upgrade MR.

Expunging of deleted messages (https://github.com/tpitale/mail_room/pull/90)

We have a lot (I believe 10s of thousands, but haven't looked recently) of deleted but unexpunged messages in our production mailbox. We should raise/implement an operational change to do a one-time manual expunge (code, but not from within mailroom) before the upgrade, as otherwise I suspect the time it might take to process the expunge could pause/break mailroom and lead to an undue outage or delays on inbound mail processing.