feat(GODT-2585): Only Start IMAP/SMTP once one user is loaded

Update ServerManager to follow the new expected behavior. The servers
will only be started when one user is active.

If all users are logged out or removed from the system, the servers will
stop.

If the network goes down, the servers will stop and resume once network
has been restored.

tests/ctx_test.go

@@ -167,6 +167,8 @@ type testCtx struct {
 
 	// This slice contains the dummy listeners that are intended to block network ports.
 	dummyListeners []net.Listener
+
+	imapServerStarted bool
 }
 
 type imapClient struct {