Request - See Mail storage by user

[ScottPeterJohnson]

(This issue was imported from Gitea) lbouriez on March 18, 2022: Hello Scott, I am seeing this: You are currently using 2.6 GiB of mail storage. But I cleaned all my mailbox (trash included...) I guess I need to give some time to the server to really clean.

But the goal here would be to allow the admin to see the space occupied by each user so we can ask them to do some cleaning Also limit the size / user could be a great addition

[lbouriez]

Related: https://github.com/ScottPeterJohnson/purelymail-issues/issues/110