Switch to using govuk_admin_template

Recommend reviewing code using`?w=1`, and doing like-for-like comparisons with preview.

* Start using admin gem which bundles jQuery, Bootstrap, environment indicator and default application layout
* Upgrade from Bootstrap 2 to Bootstrap 3
* Upgrade formtastic, formtastic-bootstrap, chosen and jQuery UI
* Use a select2 gem for assets
* Update markup to conform with newest bootstrap patterns
  * Replace grid classes on form elements with appropriate grid wrappers
  * Formtastic picks up most of the form changes, but for forms not using it:
    * Switch from `controls` to `form-group`, and ensure there's a `form-control` class on inputs
  * Add missing `-default` classes to default buttons and alerts
  * Use Bootstrap 3 kaminari themes for pagination
  * Switch to using Bootstrap 3 glyphicons
* Update flash messages for new Bootstrap 3 alerts

![screen shot 2014-08-27 at 15 04 43](https://cloud.githubusercontent.com/assets/319055/4060860/54f721b8-2df3-11e4-8032-55f619f7aff6.png)
![screen shot 2014-08-27 at 15 05 02](https://cloud.githubusercontent.com/assets/319055/4060862/54fdc43c-2df3-11e4-8ecd-a9b14caf21b6.png)
![screen shot 2014-08-27 at 15 05 37](https://cloud.githubusercontent.com/assets/319055/4060861/54fcf192-2df3-11e4-8300-c79ce51b4192.png)
![screen shot 2014-08-27 at 15 05 51](https://cloud.githubusercontent.com/assets/319055/4060863/54fdea2a-2df3-11e4-909a-078f51f1bd8f.png)
![screen shot 2014-08-27 at 15 06 05](https://cloud.githubusercontent.com/assets/319055/4060865/54ffde98-2df3-11e4-86b5-ecd4eae08317.png)