Refactor categories into rootListing

This uses proper paths with slashes everywhere, and lists documents
instead of just folders for root.

app/components/breadcrumb-nav.js

@@ -0,0 +1,4 @@
+import Component from '@ember/component';
+
+export default Component.extend({
+});

app/components/categories-nav/template.hbs

@@ -2,7 +2,7 @@
   <nav>
     <ul>
       {{#each categories as |category|}}
-      <li>{{#link-to "index" (query-params path=category)}}{{category}}{{/link-to}}</li>
+      <li>{{#link-to "index" (query-params path=category.path)}}{{category.name}}{{/link-to}}</li>
       {{/each}}
     </ul>
   </nav>