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/views/notification.scss

@@ -2,7 +2,7 @@
 
   @include core-16;
   color: $secondary-text-colour;
-  margin-top: -$gutter-half;
+  margin-top: -govuk-spacing(3);
 
   &.error {