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/tick-cross.scss

@@ -51,10 +51,10 @@
 
           position: absolute;
 
-          top: -$gutter-half - 1;
+          top: -govuk-spacing(3) - 1;
           right: 0;
-          bottom: -$gutter-half;
-          left: -$gutter-half;
+          bottom: -govuk-spacing(3);
+          left: -govuk-spacing(3);
 
           background: transparent;
         }