Add some useful page titles.

Previously, the application only ever used the world "Chessboard"
as the page title which did not really convey much information
about the page viewed.
Don’t count the OP as a reply in the topic overview of a forum.
Add read indicator to the index page.
More performant implementation of read status for entire forum.

The new implemention doesn’t query each single topic on itself,
but rather retrieves the list of read topics by this user and
compares its length to the total number of topics in the forum.
This results in constantly two queries per forum regardless of
the number of topics in the forum.
Change forum icons
Add page_topic_num configuration option.

Not used yet.
Change Forum#categorized_topics to return a hash
Add pagination to forum topics list.