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/fullscreen-table.scss

@@ -6,7 +6,7 @@
     z-index: 10;
     overflow-y: hidden;
     box-sizing: border-box;
-    margin: 0 0 $gutter 0;
+    margin: 0 0 govuk-spacing(6) 0;
     padding: 0 0 0 0;
     overflow: hidden;
     border-bottom: 1px solid $border-colour;