Update `ChatDocumentHandler` so `RoomManger` is no longer required for saving the text in the chatbar between room switches. This is achieved by allowing `ChatDocumentHandler` to get the correct `ChatBarChache` itself rather than having to have it passed from `ChatBar.qml`. This avoids any race conditions.
Use the forget function to leave a room everywhere this is both for consistency and to reduce dependencies. This way no dependency on RoomManager is required to leave a room and since in all cases they have an object they can just call the function.
