[IMP] website: use env.ref to ensure main_menu is fetched w/ correct environment

By using get_object and not passing in the request's context, creates a
divergent environment from the one used in the rest of the request,
resulting to the cached menu items not being reused when displaying the
menu in the page (during qweb rendering), saves ~4 requests on the index
page (though probably nothing elsewhere)

addons/website/controllers/main.py

@@ -34,7 +34,7 @@ class Website(openerp.addons.web.controllers.main.Home):
     def index(self, **kw):
         page = 'homepage'
         try:
-            main_menu = request.registry['ir.model.data'].get_object(request.cr, request.uid, 'website', 'main_menu')
+            main_menu = request.env.ref('website.main_menu')
         except Exception:
             pass
         else: