Allow apps to specify a full width page

* Full width pages must not be used as a default, for consistency between apps and to prevent long line lengths
* On pages with large tables the extra width can improve table legibility (although on these pages the tables should likely contain less information)
* Necessary for Panopticon index page, where content titles and slugs can get very long

![screen shot 2014-08-27 at 12 45 40](https://cloud.githubusercontent.com/assets/319055/4059079/bcd2280a-2ddf-11e4-8658-53f1a066a314.png)