mail_room: Make it possible to configure delete after delivery behavior

What does this MR do and why?

In IMAP, delete_after_delivery only marks the message as deleted, but a separate expunge_deleted step is needed to permanently remove the message.

In Microsoft Graph, delete_after_delivery soft-deletes the messages, but the message is automatically expunged after a retention period, typically 14 days. An admin can manually expunge deleted messages via the command-line.

delete_after_delivery has been set to true since 712d1768 since it wasn't necessary to configure this for IMAP, but it is necessary for Microsoft Graph since this auto-expunge behavior may not be desired. We now make this option configurable with true by default.

This requires Omnibus support: omnibus-gitlab!6605 (merged)

Relates to #386770 (closed)

Testing

With this merge request and omnibus-gitlab!6605 (merged) applied on an Omnibus instance, configure:

gitlab_rails['incoming_email_delete_after_delivery'] = false

Then run gitlab-ctl reconfigure.

With this set to true or omitted, the message is deleted:

With this set to false, the message remains in the inbox:

MR acceptance checklist

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