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
This code looks to be a repeat of https://github.com/edx/edx-platform/blob/master/common/djangoapps/student/views.py#L390. Up to you and edX if you think it's worth refactoring this into a utility method.