forms: publish url_name as slug in json webservice (#6266)

help/fr/api-schema.page

@@ -40,11 +40,13 @@ est transmis, ou à une URL <code>/json</code> autrement.
 <output>
 [{"url": "https://www.example.net/inscriptions/newsletter",
   "title": "Newsletter",
+  "slug": "newsletter",
   "count": 17,
   "authentication_required": false,
   "category": "Inscriptions"},
  {"url": "https://www.example.net/inscriptions/piscine",
   "title": "Piscine",
+  "slug": "piscine",
   "count": 6,
   "authentication_required": true,
   "category": "Inscriptions"}

wcs/forms/root.py

@@ -1002,6 +1002,7 @@ class RootDirectory(AccessControlled, Directory):
                         authentication_required = True
 
             formdict = {'title': unicode(formdef.name, charset),
+                        'slug': formdef.url_name,
                         'url': formdef.get_url(),
                         'authentication_required': authentication_required}