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)

add this:

```css
.devhub-links {
    font-size: 0;  // To strip whitespace for `inline-block` children.
    …
}
```