Add authorised statement

See 17f9dd3bbf8e043859831565064f3eb962683c71

I feel at least part of the ‘empowered aspect’ is important, even though
it’s contrary to the style guide. See comments on
alphagov/government-service-design-manual#374
Merge pull request #375 from alphagov/empowered-2-authorised

Add authorised statement