Make login and signup pages scale based on screen size.

This makes the login page look much nicer on small screens.

On a big screen it looks almost exactly how it used to (except the ends of the email and password inputs line up with the login button now):

![screenshot from 2013-08-28 18 29 25](https://f.cloud.github.com/assets/1447206/1046585/7b9d8c82-1042-11e3-982d-4508b0d231da.png)

But on a small screen it looks much better:

![screenshot from 2013-08-28 18 29 35](https://f.cloud.github.com/assets/1447206/1046588/841cb090-1042-11e3-8ed2-b953919cd1dc.png)

For reference, here's how it looks now:

![screenshot from 2013-08-28 18 33 20](https://f.cloud.github.com/assets/1447206/1046592/9d3ab9f0-1042-11e3-9156-9a0350a3d1f8.png)