[auth] Use a second authtkt policy for 'rememberme' The former solution was buggy because the expire time of the auth cookie, if set through 'remember', was lost on the first cookie reissuing. The new approach, make possible thanks to multiauth, use two different cookies. One for session bounded authentication (no 'rememberme'), and one for long lasting authentication (w 'rememberme'). The choice between the two of them is done by adding a 'persistent' argument to the top-level 'security.remember' call. Passing this argument will inhibate a policy or the other. The two policies are (a little) configurable through the 'cubicweb.auth.authtkt.[session|persistent].*' variables. Related to #4985962

[auth] Use pyramid_multiauth It makes it easier to finely tune what parts of the default authentication stack we want to use or not. It also makes it possible for any cube to add its own policy in addition to the others. Related to #4985962