[Suggestion] Board Stats - cached, saved in db

[W13]

One of my admins pointed out that we've had a 7000% increase in posting activity according to the 'Posts per week' stats in ACP.

However, that is very incorrect because all the posts we've pruned aren't taken into account! So those stats are only accurate back to the last prune.

I suggest there be a task (it can be disabled by default) that grabs the stats info (posts per day) and stores it into the special table. This way, it'd be easy to show this statistic to members as well (as it'd be pretty easy on the server).

Other things like # of member registrations can be stored as well.

Before running a prune, we can go and run the task to count member registrations and posts for today, and store it into db. - infact, that can be done by the Prune control panel itself (with a simple checkbox that asks, "Store stats info in database before prune?" or something).