Include occ command for removing orphaned accounts as a maintenance task

[skruehoetl]

Feature Request

Due to issues with the user account removal in older mail versions, orphaned accounts could be left over in the oc_mail_accounts table (probably #3135), resulting in "invisible Ghost accounts" in the mail app itself (they were not there, but the app expected multple inboxes) and errors displaying mails in the widget (see #3819).

I think, a feature allowing for a controlled/guided removal of these accounts would benefit self-made admins such as myself who are not too comfortable writing "DELETE FROM oc_mail_accounts WHERE ..." statements in SQL.

Summary

Since this is would not be a feature for regular use, I think it would best be kept as part of the "occ" maintenance tool.

As the admin of a nextcloud instance, I'd like to be able to log into the shell of my nextcloud and issue a command like "php occ mail:list-orphaned-accounts" after which the orphaned accounts are identified and listed with id, userid, server, ... , followed by "php occ mail:delete-orphaned-account 1" after which the account with the corresponding id is deleted.

... probably, in the background, there would have to be something like "DELETE FROM oc_mail_accounts WHERE id=1" but the user/admin would not have to know about SQL and the 'magic' would be the identification of the orphaned accounts. (In my case, I could only go via error messages - there's probably a smarter way to do that.)

[ChristophWurst]

For more context it's about Nextcloud accounts deleted before #3009. But I'm also not sure if every user back-end emits the user deleted event, so it doesn't hurt to have this additional cleanup command.

The rough steps are

• Iterate over all mail accounts or possibly just the grouped user_id column
• Look up the user account through the user back-end
• If the user does not exist, drop the mail account(s) of that UID