Fix #3228. Alert text for redirect from jupyter.radiasoft.org
Fix #3228. Alert text for redirect from jupyter.radiasoft.org

Shows alert text when a user is trying to log into the jupyterhublogin
app. The alert text for when they are already logged into Sirepo is handled
in PR #3230.

radiasoft/sirepo,mkeilman/sirepo,radiasoft/sirepo,mkeilman/sirepo,mkeilman/sirepo,radiasoft/sirepo,radiasoft/sirepo,mkeilman/sirepo,radiasoft/sirepo