Refactoring of qweb to call _post_processing_att for each nodes. This
change remove the crappy ovewrite in website module. The website overwrite
only the _post_processing_att to add the cdn parameters.
The static node (without t- attributes) can stay static (remove overwrite
of _is_static_node), the cdn is applied at the compile time for this
nodes instead of at the running time like the dynamic node.
Christophe Matthieu