TF-1437 Only show subscribed mailbox in destination picker

model/lib/extensions/list_presentation_mailbox_extension.dart

@@ -0,0 +1,8 @@
+
+import 'package:model/mailbox/presentation_mailbox.dart';
+
+extension ListPresentationMailboxExtension on List<PresentationMailbox> {
+
+  List<PresentationMailbox> get listSubscribedMailboxes =>
+    where((mailbox) => mailbox.supportedSubscribe).toList();
+}