Fix a couple of issues with closing channels.

- GroupChatManager never subscribed to the event bus
- Comparing Optional<Connection> to Connection fails

Closes #782

src/main/java/com/dmdirc/GroupChatManagerImpl.java

 
     @Handler
     void handleChannelClosing(final ChannelClosedEvent event) {
-        if (event.getChannel().getConnection().equals(connection)) {
+        if (connection.equals(event.getChannel().getConnection().orElse(null))) {
             final GroupChat channel = event.getChannel();
             connection.getWindowModel().getInputModel().get().getTabCompleter()
                     .removeEntry(TabCompletionType.CHANNEL, channel.getName());

src/main/java/com/dmdirc/Server.java

 
         highlightManager = new HighlightManager(windowModel);
         windowModel.getEventBus().subscribe(highlightManager);
+        windowModel.getEventBus().subscribe(groupChatManager);
         windowModel.getEventBus().subscribe(this);
     }
 
             synchronized (myStateLock) {
                 eventHandler.unregisterCallbacks();
                 windowModel.getConfigManager().removeListener(configListener);
+                windowModel.getEventBus().unsubscribe(groupChatManager);
                 windowModel.getEventBus().unsubscribe(highlightManager);
                 executorService.shutdown();