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)

where did this colour come from? I took phil's [`$primary-blue`](https://github.com/mozilla/fireplace/blob/a82a010/src/media/css/lib/variables.styl#L59) and darkened it by 10% in [`pretty-select.styl`](https://github.com/mozilla/fireplace/blob/ca58cd3/src/media/css/pretty-select.styl#L5) to come up with #2ba3ff.

## $primary-blue                               darken($primary-blue, 10%)
<a href="http://www.colorhexa.com/4cb1ff.png"><img src="http://www.colorhexa.com/4cb1ff.png" height="20%" width="48%"></a>    <a href="http://www.colorhexa.com/2ba3ff.png"><img src="http://www.colorhexa.com/2ba3ff.png" height="20%" width="48%"></a>

vs.

## $primary-blue                               $primary-blue-highlight
<a href="http://www.colorhexa.com/4cb1ff.png"><img src="http://www.colorhexa.com/4cb1ff.png" width="48%"></a>    <a href="http://www.colorhexa.com/42cafe.png"><img src="http://www.colorhexa.com/42cafe.png" width="48%"></a>