Replace $gutter with govuk-spacing function

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`.

app/assets/stylesheets/components/show-more.scss

@@ -4,7 +4,7 @@
   @include core-16;
   display: block;
   padding: 0 0;
-  margin: $gutter-half 0 $gutter-half 0;
+  margin: govuk-spacing(3) 0 govuk-spacing(3) 0;
   text-align: center;
   border-top: 1px solid $border-colour;