xhtml_style/authentication_policy: merge "logged_in" implementation
This simplify code and also bring the functionnality of c484f8aa (erp5_xhtml_style Base_cancel and logged_in: do not allow redirection outside ERP5 site., 2016-02-12) for the cases where authentication_policy is installed.
This also fixes a problem with translations of "Your password will expire at {date}", which was using different messages for every possible date.
Tests needed to be updated because we now redirect with properly URL encoded parameters.