Updated stay signed in system for a real (working) system

Thanks to Sbgodin for his work on Shaarli and Leed, which I adapted.

Please note that if you already have a working instance of BouffeATUlm,
you have to update manually your database. You have to set a new field
for User database (at the end of the database). This field has to be
called "stay_signed_in_token" and to be VARCHAR(32).
Wrong values for the stay signed in cookies at regeneration... :/