User: refactoring of webdriver and login sequence

With this, a few quasi-loops between the Heptapod class, the User class and the session module get simplified.

Actual tests code can now just rely on User.webdriver and get guaranteed to obtain a working web driver in which the given user is authenticated. This is still fully lazy (sign-in sequence takes time), and closes also some remaining holes in the web driver closing logic (leaking of big fat, chromedriver processes).