Do not show "Mark topics as read" when there are no topics

The parent could not be easily fixed because the code surrounding the mark topics link differs between the top and bottom of viewforum_body.html.

Copied from: https://github.com/phpbb/phpbb3/pull/919#issuecomment-12567979
There are a few differences.
TOTAL_TOPICS instead of the .topicrow you added
TOTAL_TOPICS is being displayed in the output if not TOTAL_POSTS (and checks both against "and not NEWEST_USER." I have no idea why this is being done)
PAGINATION has a if PAGE_NUMBER wrapper around it on the second instance.

To properly fix this, I think we need to do a few things.
First, we need to figure out where the two sets of code diverged and why that happened.
Second, we need change the block so that they are the same for both.