Restrict Access to Creating Mailboxes

[CodeShellDev]

As proposed by @Jniklas2 in #15 we should add a way for disabling Mailbox creation and instead only allow permitted Users to add mailboxes (admins could then manually add mailboxes to Users).
To percent Users from literally claiming any email.

[CodeShellDev]

Would adding an env var flag for this and then checking for a "mailbox-admin" Claim from the IdP (i.e. authentik) suffice @Jniklas2?