The central part of the board - topic display.
This function loads the posts in a topic up so they can be displayed.
It uses the main sub template of the Display template.
It requires a topic, and can go to the previous or next topic from it.
It jumps to the correct post depending on a number/time/IS_MSG passed.
It depends on the messages_per_page, defaultMaxMessages and enableAllMessages settings.
It is accessed by ?topic=id_topic.START.