add header to user registration (Fix for #3643) [ci skip]

Here's the final fixes, from the previous PR here: https://github.com/diaspora/diaspora/pull/3720

Would love to get this merged ASAP, so that I can start working on the other half of the original PR that I wanted to work on, which was unifying some of Diaspora's UI stuff and looking into making things more modular.

This was proposed in #3643 as an alternative for a cancel button.

Here's what it looks like!
![header](http://i.imgur.com/aoWvO.png)