`User.onLogin()` shall be called earlier (BREAKING v3)

[phorward]

This is a breaking issue as reminder for ViUR4.

#1573 already faced this in v3.8 but it was a breaking and too big change.
Projects requiring #1573 must therefore resolve it like so:

    def onLogin(self, skel):
        # Synchronize user with external system
        if not skel["manualoverride"] and skel["debitor"]:
            ok, _ = conf.main_app.thirdparty.sync_user_skel(skel)
            if ok:
                skel.write(update_relations=False)

                # Is user still active, or meanwhile flagged locked?
                if not self.is_active(skel):
                    session.killSessionByUser(skel["key"])
                    raise errors.Redirect("s/user_locked")

                # FIXME: HOTFIX IN COMBI WITH VIUR3 authenticateUser;
                # SOME ALREADY SET SESSION VALUES MUST BE SET AGAIN...
                session = current.session.get()
                session["user"] = skel.dbEntity

                current.request.get().response.headers[securitykey.SECURITYKEY_STATIC_HEADER] = \
                    session.static_security_key
                current.user.set(self.getCurrentUser())
                # FIXME: ^^^--- UNTIL HERE ---^^^

        # Super-Login
        super().onLogin(skel)