Register button on devhub (bug 1095659)

WIP: still some touchups to do.

![sign-in-up mov](https://cloud.githubusercontent.com/assets/211578/4961774/fe1abd78-66d2-11e4-9fd9-17f66867526d.gif)

I believe the cookies are cleared by django on line 202 with `auth.logout(request)`. I'll add a test if there isn't one.