[web] Move request handling logic into cubicweb application. (closes #2200684) We improve http status handling in the process: ``application.publish`` have been renamed to ``application.handle`` to better reflect it's roles. The request object gain a status_out attribute to convey the HTTP status of the response. WSGI and etwist code have been updated. Exception gain status attribute

[login] split authentication logic from post authentication logic (closes #2200755) * The Session manager is now only in charge of providing a valid session. * LoginControllers are now used in all case but wrong credential. * The LoginControllers are in charge of redirecting the user to the page wanted to see in the first place, expected to see. * The login form is now always submitted to the login controller with an extra argument pointing to the url we should redirect too after successful authentication. The ``"log out first logic"`` logic on login controller is removed because: 1. Other web actor do not do that. 2. Removed code do not need to be reimplemented. 3. We can only get it to work again in a single case: use do a GET request on http://www.my-cw-stuff.io/login 4. I do not see it's purpose.

[LoginForm] refactor to ease other implementation An Abstract BaseLoginForm is created. It is intended to be used by cubes who want to implement new login form. It gather common login form elements in particular the form action. This will ease redefinition of this form action in later commit.