Make login UI less broken for really small screens and really big screens

Galaxy S II

![image](https://cloud.githubusercontent.com/assets/464123/4944736/7e294774-6600-11e4-8dc6-2702b399c7c0.png)

Nexus 9

![image](https://cloud.githubusercontent.com/assets/464123/4944755/9a07b4a8-6600-11e4-9342-6c9994e8fb6c.png)