Fix session closing for cubicweb 3.19 Related to #4291173

Convert cubicweb.NotFound to HTTPNotFound Related to #4291173

Use registry['cubicweb.registry'] instead of registry['cubicweb.appli'].vreg because the application may not be present. Related to #4291173

Use a predicate based view selection for handling /login This will allow easy addition of login handlers from the application or cubes Related to #4291173

Use a tween application instead of a catchall route. Using a catchall route has some drawbacks. Especially, we have no mean to have a route that would match only if no other one does AND no view matches either. Said differently, our default handler cannot be plugged on the route level nor the view level, because it is has to be activated only if nothing else works in the pyramid application. Using a tween application allow to handle requests that raises a HTTPNotFound error, while having the pyramid error handler still active between our tween app and the outside world. Related to #4291173

Document the view problem hypothesis. Related to #4291173

If the postlogin_path is 'login', redirect to '/' instead Related to #4291173

Put the login view in a separate module. Related to #4291173

Separate into 4 modules * init_instance: load the cubicweb repository from the `pyramid_cubicweb.instance` configuration key * defaults: provides cw-like defaults for the authentication and session management * core: make cubicweb use the authentication and session management of pyramid. It assumes the application provides the auth policies and session factory, and that the `cubicweb.*` registry entries are correctly initialised. This is this only required module or pyramid_cubicweb, the other ones are optional if the application provides its own versions of what they do. * bwcompat: provides a catchall route that delegate the request handling to an old-fashion cubicweb publisher (ie using url_resolver and controllers). Related to #4291173

Update the TODO list Related to #4291173