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/template.scss

@@ -9,7 +9,7 @@
   position: absolute;
   background: $link-colour;
   color: $white;
-  padding: 10px $gutter-half;
+  padding: 10px govuk-spacing(3);
   z-index: 10000;
 
   &:link, &:visited {