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/checkboxes.scss

@@ -3,7 +3,7 @@
   .selection-summary__text {
     @include core-19($tabular-numbers: true);
     padding: 5px 0 0 0;
-    margin-bottom: $gutter / 2;
+    margin-bottom: govuk-spacing(3);
 
     &:focus {
       outline: none;
@@ -66,7 +66,7 @@
 }
 
 .selection-content {
-  margin-bottom: ($gutter / 3) * 2;
+  margin-bottom: govuk-spacing(4);
 
   .checkboxes-nested {
     margin-bottom: 0;