Replaced `$gutter` and similar variables such as `$gutter-half` with the
`govuk-spacing()` static spacing function. This uses `govuk-spacing()`
instead of `$govuk-gutter` because `$govuk-gutter` should only be used
for the gaps in between grid columns and we were mostly using `$gutter`
to add more space around elements.
There are other places in the SCSS files where we had hardcoded a
measurement in px which could be replaced with `govuk-spacing`, but this
commit only replaces the existing uses of `$gutter`.
