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/letter.scss

@@ -9,7 +9,7 @@ $iso-paper-ratio: 141.42135624%;
 .letter {
 
   padding: $iso-paper-ratio 0 0 0;
-  margin: 0 0 $gutter 0;
+  margin: 0 0 govuk-spacing(6) 0;
   position: relative;
   background: $panel-colour;