Add caching by url.path to rigel_preprocess_breadcrumb()

Problem/Motivation

• The rigel_preprocess_breadcrumb() appends the page title to the end of the trail. In Drupal, breadcrumbs are cached based on the parent breadcrumb item.
• So if the path is: Home / Parent 1 / Page title, the breadcrumb is cached from Home / Parent 1. Then if we go to another page that looks like Home / Parent 1 / Page title 2, since the caching happens at Parent 1, we'll see Home / Parent 1 / Page title instead of Home / Parent 1 / Page title 2
• To get around this because we have Page title in the breadcrumb trail, we need to set the following $variables['#cache']['contexts'][] = 'url.path'; in rigel_preprocess_breadcrumb()
• This changed happened in Drupal 8: https://www.drupal.org/node/2699627. To make sure we don't blow up the cache too much, we should only set the cache context if the page isn't a 404.