External links update

Changing the external link icon. (It is currently the icon for 'opens in a new window'.)

https://www.pivotaltracker.com/story/show/44025523

This included naming the existing icons better & moving the CSS into https://github.com/govuk_frontend_toolkit (with @2cb594a8afb7b04ad2cd1a0d581a3be359170cde)