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 will break the redirect flow @marcotuts wanted during login in the LMS. If the systems need different redirect URLs on successful login, suggest introducing a seam for customization and setting this value like other system-specific configuration vars.

Is there not a dasboard in the CMS? If not, how do users manage their account associations?