Interesting comments

Fulfills:
https://code.google.com/p/lesswrong/issues/detail?id=399
https://code.google.com/p/lesswrong/issues/detail?id=400

I've added the concept of a leading comment - leadingness is comment karma plus one half the karma of all descendants.  Leading comments can be viewed in the new category dashboard - either all comments or comments on posts a user has subscribed to.  The dashboard views by default show comments since the last time a user has viewed them, with visits occuring within 12 hours of each other considered the same, but the standard time increments (hour, day, week...) are also possible.