mail-server/dovecot.nix: automatically subscribe to new mailboxes

When a mailbox is created by sieve or is delivered to directly by IMAP commands,
created IMAP folders are not subscribed to by dovecot.  These configuration
options change that.

Acked-by: Ruben Maher <ruben@maher.fyi>

mail-server/dovecot.nix

@@ -111,6 +111,9 @@ in
         plugin {
           sieve = file:/var/sieve/%u.sieve
         }
+
+        lda_mailbox_autosubscribe = yes
+        lda_mailbox_autocreate = yes
       '';
     };
   };