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/textbox.scss

@@ -36,7 +36,7 @@
     overflow-wrap: break-word;
     word-wrap: break-word;
     border: 2px solid transparent;
-    padding-bottom: $gutter-half;
+    padding-bottom: govuk-spacing(3);
     z-index: 10;
 
     .placeholder,