use correct icon sizes instead of relying on background-size to scale up hmrc icon.
use correct icon sizes instead of relying on background-size to scale up hmrc icon.

resolves #131

tmikus/assets-frontend,alexanderjamesking/assets-frontend,xnejp03/assets-frontend,tmikus/assets-frontend,alexanderjamesking/assets-frontend,alexanderjamesking/assets-frontend,xnejp03/assets-frontend,hmrc/assets-frontend,tmikus/assets-frontend,hmrc/assets-frontend,xnejp03/assets-frontend,hmrc/assets-frontend