streamlined shibboleth logout process

Originally, I just set out to remove unnecessary JS from the logout template (refs #404).
But then I came to realize that that page is a complete waste, since it only acts as a stepping stone to redirect user to the external logout page if Shibboleth authentication is being used.
Why this was implemented like this in the first place is puzzling to me, not sure if this is a remnant of an earlier version of Ilios that was using framesets.

Anyhow, I yanked the logout template altogether and moved the redirect into the controller function where it belongs.

This change only affects Ilios instances using Shibboleth auth. so you'll only have to test logout on these.