Rename world location description => mission statement.

In the public interface, the information is presented under the
title 'Our Mission'.  Calling it the description in the admin
interface and model is inconsistent.

Bonus feature: Added xss protection to the mission statement
Merge pull request #187 from alphagov/rename-world-location-description-to-match-public-interface

Rename world location description => mission statement.
Remove baldy copied CSS

Please don't copy large chunks of CSS around. Most of this was
irrelevent for the page it was being copied to and it is not the right
way of working.