Remove unused AUTH_CONFIG
Split Registration into Web_site and Web_main
Rename Templates into Web_templates

All modules in src/web start by either "web_" or "auth_".
All services are relative to path defined in Web_main
Auth_cas: factorize URL computation
Remove CONT_SERVICE argument of AUTH_SERVICE

 - rename AUTH_INSTANCE into AUTH_HANDLERS
 - add "logout" to it
 - rename existing "handler" into "login"
 - logged_user contains AUTH_HANDLER instead of CONT_SERVICE
Make logged_user private to Auth_common
Simplification
Rename Auth_common into Web_auth

Keep "Auth_" prefix for auth systems.