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
There's no guarantee a particular edx-platform deployment will have both CMS and LMS, so having the UX only in the LMS may not be sufficient. I'm also not familiar enough with the details of how accounts work in both systems to know if this is a practical solution. edX folks, what do you think?