Fix spacing a different screen widths
Fix spacing a different screen widths

This changes the `l-container` class to use the `site-width-container`
mixin that is provided and provives the spacing based on screen
width for other parts of the layout, like the header.

uktrade/data-hub-frontend,uktrade/data-hub-frontend,uktrade/data-hub-fe-beta2,uktrade/data-hub-frontend,uktrade/data-hub-fe-beta2