Implement unread posts

[eevee]

Need some planning here; most likely I will do something like this:

• make a table of users and threads(/posts) they have read or not read, depending on which takes less space
• keep track of the last time each user looked at the main forum index or the recent-posts page or whathaveyou, and use that to create a divider bar
• X amount of time before that last look (or the present?), assume everything is read and clear the mapping table as appropriate
• of course, mark any thread (or post?) the user sees as read

Also I should create a page called UnreadPosts or something.

[eevee]

Tee-hee, wiki markup is hard.

[eevee]

My notes tell me:

Any thread with lastpost.time BEFORE thread_view_cutoff is considered read. Otherwise, any thread NOT in the thread_views table at all is considered unread. Otherwise, any thread in the thread_views table is considered read IFF last_viewed > lastpost.time

READ: lastpost.time <= user.thread_view_cutoff OR (thread_views.last_viewed IS NOT NULL AND thread_views.last_viewed > lastpost.time) NOT: lastpost.time > user.thread_view_cutoff AND (thread_views.last_viewed IS NULL OR thread_views.last_viewed < lastpost.time)

[eevee]

Beginnings done by r287. Still to do:

• Actually set thread_view_cutoff, both via a 'mark as read' link (which would work differently for a subforum vs globally.. ugh) and expiration
• Show unread marker on the forum overview level
• Evaluate that <col> stuff on the front page, and either style it and use it globally or toss it
• lol I forget it's late

[eevee]

Fixed a huge problem in r294.

Relatedly, I think:

• what the forum view on the front page does should be moved to its own New Posts page or something; figure out exact mechanics sometime
• front page, if anything, should just have newly created *threads*; current display is confusing at best

[eevee]

Forums now have unread post markers and speed was greatly improved in r386.

Other issues split off into #203, #211, and #212.