[twig] URL Handlers

[bruno]

Add url handlers amongst all admin page, and avoid any hardcoded link inside twig template files. This handler can also be used on public part, if needed.

See  https://docs.djangoproject.com/en/dev/ref/templates/builtins/#url for instance in django

examples :

• {% url 'admin.blog.post' {id:1} %} to retrieve url that enables to edit post id#1
• {% url 'admin.blog.posts' {selected:1} %} to display posts list, filtered on selected posts

nota : a new tag is not necessary, that could be performed through a function, ie. {{ url('admin.blog.post',{id:1}) }}

[bruno]

could be integrated as part of #1645

[Dsls]

(In [e8b17b3a7413]) 1st attempt for admin url handler. See #1645, see #1959

[Dsls]

(In [519de2d7e4fb]) menu items deserve adminurl abstraction too, See #1645, see #1959