ShowWidget: Restore sensitivite of the unsub button.

If you clicked unsub, then undo and then the same show widget you
would navigated to the previous ShowWidget and the unsub button
would still be insensitive.

hammond-gtk/src/widgets/show.rs

@@ -145,6 +145,7 @@ fn on_unsub_button_clicked(pd: Arc<Podcast>, unsub_button: &gtk::Button, sender:
     };
 
     wrap().map_err(|err| error!("Action Sender: {}", err)).ok();
+    unsub_button.set_sensitive(true);
 }
 
 fn on_played_button_clicked(pd: Arc<Podcast>, episodes: &gtk::Frame, sender: Sender<Action>) {