Added a new Handlebars helper to loop around every nth items in a group.
From http://funkjedi.com/technology/412-every-nth-item-in-handlebars/
Used the new {{#grouped_each}} Handlebars helper to create a new navigation <ul> every 20 pages (i.e. Sass partials).
Updated navigation theme styles to display multiple navigation <ul> as columns across the page.
Combined with the change to the template, this solves the problem that the navigation disappears off the bottom of the page when there are a large number of partials.