Upgrade Bootstrap to 2.3.2

* Move from Bootstrap 2.0.3 to the latest version of Bootstrap 2, 2.3.2
  * https://github.com/twbs/bootstrap/releases
  * Whitehall now has an accurate Bootstrap guide to refer to: http://getbootstrap.com/2.3.2/
* Use a gem to provide assets as well as Bootstrap mixins and variables
* Remove old bootstrap assets
* Customise Bootstrap by downsizing the default font size and padding to match what was there before
* Simplify govspeak styles and add styles to cope with new `h3` default margins and font-size
* Remove `list-inline` patch, it comes with Bootstrap 2.3.2

When reviewing this PR, I highly recommend running the branch on a VM and doing many like-for-like comparisons with preview. Everything should look and behave the same. With the exception that page headers are slightly larger now.

![screen shot 2014-08-15 at 15 47 07](https://cloud.githubusercontent.com/assets/319055/3934425/17d2f328-248b-11e4-9725-7ca1c52d536a.png)