Instead of redirect on signup form validation, render new

Keeps submitted data. Fixes #4663.

However some problems.

1) The URL changes to /users since that is the POST url. This doesn't affect usability of the form as far as I can see (except maybe on an F5..). Is there some way to modify the routing system to take in registrations#create at /users/sign_up? Not sure I understand the Rails routing system especially with the Devise stuff on top.

2) Had to remove the `only: [:new]` from the `before_filter` as otherwise the CSS was not included correctly. Why and is this ok? :P 

Screenshot;
![pic](http://i.imgur.com/GKZdKc1.png)

Ps. Tests will fail I think, will fix once issues cleared up.