Update layout.scss
Update layout.scss

Use new grid layout mixins from the front end toolkit to define:
- A single page container
- A grid row for grid columns to sit in
- Grid column widths

alphagov/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,TFOH/govuk_elements,gavboulton/govuk_elements,joelanman/govuk_elements,gavboulton/govuk_elements,TFOH/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,alphagov/govuk_elements,joelanman/govuk_elements