Fix for Shibboleth "visit IdP twice" bug

Authentication was occurring the first time the user selected their
institution, but they were then being redirected back to the IdP by the
“sign_in_and_redirect s_user, event: :authentication” line. The second
time they selected their institution, they were already authenticated
and were simply redirected. This fixes that, sending the user to the
root path after logging in via Shibboleth or to the edit profile page
after linking their account.