Comment 10 for bug 1917083

I don't think I should commit this since I did the rebase. Here's a test plan:

1. Apply the fix.
2. Make sure native OPAC login still works correctly via both the login modal and /eg/opac/login.
3. Make sure OPAC logout also works correctly.
4. Set the opac.login.shib_sso.enable org setting to 'true'. OPAC login should now present a "Login with SSO" button instead of the usual login form; clicking it should redirect you to /Shibboleth.sso/Login.
5. Set the opac.login.shib_sso.allow_native org setting to 'true'. OPAC login should now present both a "Login with SSO" button (which should work as in step 4) as well as the native EG login form (which should work as usual).
6. Make sure that clicking "Logout" ends your EG auth session and redirects to /Shibboleth.sso/Logout.

You don't technically need a working Shibboleth install to follow this test plan, you'll just get a 404 error when you're redirected to the Shibboleth.sso pages.