Make sticky nav display widths filterable

[benlk]

In many places in js/navigation.js there are lines like this:

    if ($(window).width() > 768) {
      this.stickyNavTransition();
    }

These 768s should not be hardcoded, but rather drawn from the Largo javascript object created in inc/enqueue.php and should be made filterable, with code similar to this:

        wp_localize_script(
            'largoCore', 'Largo', array(
            'is_home' => is_home(),
            'is_single' => is_single() || is_singular(),
            'sticky_nav_options' => array(
                'sticky_nav_display_article' => (bool) of_get_option( 'sticky_nav_display_article', 1 ),
                'main_nav_hide_article' => (bool) of_get_option( 'main_nav_hide_article', 0 ),
                'nav_overflow_label' => of_get_option( 'nav_overflow_label', 'More' ),
                'sticky_nav_breakpoint' => apply_filters( 'largo_sticky_nav_breakpoint', 768 )
            )
        ));