CLC-3215: Allow deep linking into CalCentral - Second Try

CLC-3215: Allow deep linking into CalCentral

https://jira.ets.berkeley.edu/jira/browse/CLC-3215

Task: In order to support linking from Cal Student Central or Bear
Facts directly to My Academics or My Finances, we need to be able
to pass the correct URL to CalNet so that the user can go directly
to those pages in CalCentral.

Fix:
1. userService.signIn - capture the full desired path on the client-side
2. SessionsController::new - Pass the "service" parameter to our
   OmniAuth provider for CAS (/auth/cas?service=<service>)
3. SessionsController::continue_login_success - in the OmniAuth CAS
   callback, consider the service parameter when making a successful
   authenticated redirect

This is a second try at this PR.  The first time, I used a parameter named smart_path with only a relative url, which worked locally for me, but broke authentication on calcentral-dev.

Now using the CAS server's documented "service" parameter instead of the undocumented "smart_path" parameter that I'd invented myself.