Shedna/enable third party auth on studio

@talbs can you check for the buttons' style ?
@johncox-google if you wanna take an eye on this ?
cc @nedbat
To the best of my knowledge this is a real problem. This URL is used to redirect the user after a successful login. See http://psa.matiasaguirre.net/docs/configuration/settings.html. There are lots of paths the user can take through the various auth flows, so it's possible your manual testing didn't hit the ones that require this value.

It looks like you've got some test failures: https://jenkins.testeng.edx.org/job/edx-all-tests-auto-pr/13132/.

Why is going to /dashboard in CMS undesirable? Is there no dashboard? If so, where are account associations managed in CMS?