Update GOV.UK template from 0.6.2 to 0.7.1

This includes an up-to-date copy of GOV.UK's footer styles,
which keeps us looking consistent.
Remove media-down mixin

In this case, this mixin does exactly the same thing as the frontend
toolkit mixin. Remove ours in preference of the toolkit one.
Merge pull request #435 from alphagov/update-govuk-template-0-7-1

Update GOV.UK template to 0.7.1