backport stable

[web session] fix session handling so we get a chance to have for instance the 'forgotpwd' feature working on a site where anonymous are not allowed fix several pbs: * we need a session id and a session cookie anyway, else subsequent http queries are unrelated * this imply some changes in the session attribution workflow for session without a cnx * some views/selectors must be fixed for cases where session has no cnx On the way, avoid unnecessary Redirect on successful login. closes #750543

[ajax js/css] modconcat fix: load code exactly once (auc, fcayre) * introduces cw.ajax namespace (currently hosts only recent modconcat functionality) * handle load-one/load-many-at-once edge case * properly handle already-loaded js resource * avoid .appendTo to trigger script evaluation since from jquery 1.5 this results in uncached ajax calls