Style the not-available page

This page may show up a lot in our beta period if people share links to their beta maps, which can not be made public.

Before:
![2013_10_18_111113_1075x436](https://f.cloud.github.com/assets/17363/1361452/21c60ba6-3808-11e3-93df-72440fdea21f.png)

After:
![2013_10_18_111312_1072x377](https://f.cloud.github.com/assets/17363/1361456/27f2d28e-3808-11e3-8949-7b130de17d77.png)