Adds rankings page

Now the Rankings button on the home page goes to a rankings page that lists users with an auto-generated user name, number of puzzles conquered, and average difficulty of puzzles conquered.

It does not yet actually put the users in order according to their puzzle conquering statistics: my attempt at this can be seen commented out on line 12 of puzzles_controller.rb, and that did not work. However, the plan is definitely to come back to this.