compose: Add a controller.dispose call that we forgot

We do this at the other sites where there's an existing controller
that we want to replace with a new one. I suppose this fixes a small
memory leak; I just noticed it from reading code, not from profiling
or anything.

Author: chrisbobbe

lib/widgets/compose_box.dart

@@ -1989,6 +1989,7 @@ class _ComposeBoxState extends State<ComposeBox> with PerAccountStoreAwareStateM
     final store = PerAccountStoreWidget.of(context);
     final outboxMessage = store.takeOutboxMessage(localMessageId);
     setState(() {
+      this.controller.dispose();
       _setNewController(store);
       final controller = this.controller;
       controller