Create Mailu PoC on OSUOSL infra

Go live checklist:

[ ] Get signoff from others on workflow breakage
[ ] ~Ditch geerlingguy.docker Ansible dep~ out of scope
[x] Rebuild the server from scratch on the prod domain
[ ] ~Generate DKIM keys (must be done manually in the admin interface)~ we're not going to do outbound ourselves
[ ] Actually associate a floating IP https://github.com/SeaGL/seagl-terraform/issues/75
[ ] Create postmaster user/alias (https://mailu.io/master/compose/setup.html#start-mailu)
[ ] Port over config changes from https://github.com/SeaGL/seagl-ansible/pull/26 (partially done; see https://github.com/SeaGL/seagl-ansible/issues/129)
[x] Figure out monitoring
[ ] Switchover DNS, etc.
[ ] Fix api-senders@seagl.org as used by OSEM (apparently this can only be used to spoof existing mail addresses)
[ ] Users have to manually create an Archive folder and mark it as such in Roundcube
[ ] Update volunteer interest form with Mailu intake info

Minimal individual user go live checklist:

[x] Route inbound email to Mailu
- "To set up other email delivery options, you must first add mail routes for your on-premise servers using the Add Route setting." (source)
[x] Set up rspamd external relay plugin to process messages forwarded through Google https://github.com/SeaGL/seagl-ansible/pull/127
[x] Route outbound mail to Google SMTP relay https://github.com/SeaGL/seagl-ansible/pull/126
[x] Set up Borg (https://github.com/SeaGL/seagl-ansible/issues/119) https://github.com/SeaGL/seagl-ansible/pull/125
[x] Merge https://github.com/SeaGL/seagl-ansible/pull/88
[x] Figure out what we can't change later
[x] Create users for non-Google folks
[ ] Create Google user inboxes, sync Google inboxes and delete Google users

Breakage discovered during migration:

[x] Mailu users can't email Google @seagl.org users
[ ] Google Group memberships change to use user-migrated@seagl.org
[x] Current migration procedure causes invite mail to send to the Mailu inbox, not the Gmail inbox
[ ] Deal with AJ, who already has a -migrated email and therefore out-of-sync listserv email (and also Norm who needs one no matter what)
[ ] Google Groups aliases, such as matrix-help@ -> sre@, aren't included in the Mailu-to-Google-address routing fix

Easy wins:

SeaGL / organization