refactor: Logging (chat logger) (#5058)

It's no longer a singleton

It's now a unique_ptr that dies together with the Application

* Add getChatLogger to EmptyApplication

* unrelated change: Access Application::instance statically

* fix logging init order

* Add changelog entry

src/common/Channel.cpp

@@ -101,7 +101,8 @@ void Channel::addMessage(MessagePtr message,
         {
             channelPlatform = "twitch";
         }
-        app->logging->addMessage(this->name_, message, channelPlatform);
+        getIApp()->getChatLogger()->addMessage(this->name_, message,
+                                               channelPlatform);
     }
 
     if (this->messages_.pushBack(message, deleted))