improve json category REST API, sort and include description (#4958)

help/fr/api-schema.page

@@ -78,7 +78,8 @@ La liste des catégories est disponible à l'URL <code>/categories</code>.
 {"data":
   [
    {"url": "https://www.example.net/inscriptions/",
-    "title": "Inscriptions"},
+    "title": "Inscriptions",
+     "description": "&lt;p&gt;Pour vous et vos enfants...&lt;/p&gt;" },
    {"url": "https://www.example.net/etat-civil/",
     "title": "État civil"}
   ]

wcs/forms/root.py

@@ -1005,9 +1005,15 @@ class RootDirectory(AccessControlled, Directory):
         user = get_user_from_api_query_string() or get_request().user
         list_categories = []
         charset = get_publisher().site_charset
-        for category in self.get_categories(user):
-            list_categories.append({'title': unicode(category.name, charset),
-                                    'url': category.get_url()})
+        categories = self.get_categories(user)
+        Category.sort_by_position(categories)
+        for category in categories:
+            d = {}
+            d['title'] = unicode(category.name, charset)
+            d['url'] = category.get_url()
+            if category.description:
+                d['description'] = unicode(category.description, charset)
+            list_categories.append(d)
         get_response().set_content_type('application/json')
         return json.dumps({'data': list_categories})