mail_room: Allow configuration of delete_after_delivery parameter

What does this MR do?

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 by default since gitlab@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 GitLab Rails support: gitlab!108494 (merged)

Related issues

Relates to gitlab#386770 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes: see gitlab!108494 (merged)
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened: gitlab-org/charts/gitlab!2906 (merged)