[Feature: Pro Version] Add option to set max. accounts and max. quota for domainadmin's accounts

[Lednerb]

It would be a nice feature that a system admin can limit the max quota for the accounts created by a domain admin.

Maybe that the system admin can set 1 GB as a maximum size per account and also that the domain admin for example.com can only create 10 email accounts.

The number of redirects can be unlimited because for a server admin / system admin this should not be a problem but maximum disk size and amount of users can be a problem.