Calculate avg more efficiently

[mjip]

Right now, when the average is recalculated, it'll loop through every entry. We don't have too many entries right now, but this will get inefficient fast. The DB needs to be updated with the count of how many puzzles someone has done (regular + saturday), and the avg could then be recalculated with https://math.stackexchange.com/questions/106313/regular-average-calculated-accumulatively. Related to #3, since both require updating the DB to keep track of counts.