don't display the sticky nav at the top of the page unless it's appropriate

[benlk]

Changes

• Creates new method stickyNavScrollTopHide on the Navigation prototype in js/navigation.js that removes the show class if the window is scrolled too far up.
• Separates the cases in Navigation.prototype.stickyNavResizeCallback. The new case groupings are:
  • It's a mobile device: $(window).width() <= 768 , where the sticky nav should initially show at all scroll positions.
  • Configuration options: Largo.sticky_nav_options.sticky_nav_display || ( Largo.sticky_nav_options.main_nav_hide_article && ($('body').hasClass('single') || $('body').hasClass('page')) ), where the sticky nav should not show if the scrollTop is above the main nav.
  • The default case, where the sticky nav should not show.
• On the scroll event callback Navigation.prototype.stickyNavScrollCallback, prevent the show class from being added to the sticky nav if the page was not previously scrolled. This is needed because Navigation.prototype.stickyNavScrollCallback is called on Navigation.prototype.bindStickyNavEvents is called onNavigation.prototype.bindEventsis called onNavigation.prototype.init` is called when the page is loaded.

Why

After #1262, the sticky nav appears at the top of the page on initial page load, when it should not appear until farther down the page (after scrolling past the main nav). This is part of #1260.

Notes

This PR adds some in-file docs for changes made in js/navigation.js in this PR, but the file in general needs a lot more documentation. Even something like this would be helpful in the future:

An list of what in Navigation calls what
var Navigation = function() {
  return this.init();
    this.enableMobileDropdowns();
    if ($(window).width() > 768) {
      this.stickyNavTransition();
    }   
    this.bindEvents();
      this.bindStickyNavEvents();
        this.stickyNavResizeCallback();
          this.stickyNavSetOffset();
          this.stickyNavTransitionDone();
        this.stickyNavSetOffset();
    this.responsiveNavigation();

[aschweigert]

this seems fine to me