Redirect on Session Expiration

When a user's session expires and `SimplesamlSubscriber::checkAuthStatus` is called it kicks off the `user_logout` flow. That flow makes its way to `SimplesamlphpAuthManager::logout` which hardcodes the `redirect_path` to `base_path()`. The issue here is that this prevents navigation with an expired session by requiring users to bounce to the `base_path()` before going where they intended. A user case is:

* user logs in, through SAML on Monday and does a bunch of work
* leaves their session open, but walks away from the computer for the evening
* returns on Tuesday (after their session has expired) and clicks a link in their email to `/about`
* now, the Saml module gets involved and notices the session has expired
* the logout flow is run when `/about` is loaded
* the user is redirected to `base_path()` and never sees `/about`

This is more catastrophic if a user is clicking a "vanity" URL/redirect. If they come in on Tuesday and try to get to `/vanity` they'll be redirected to the homepage and not the intended redirect.

I would suggest removing `$redirect_path = base_path();` from the logout flow, but I'm not sure how that would affect actual logouts since our Saml provider hijacks logouts anyway.