Set vertical spacing for <main>

GOV.UK Frontend gives `<main>` different padding
on the top and bottom to us.

This adds our spacing to `<main>` for pages
without a side nav (except the product page which
is special) and to the row containing the `<main>`
and side nav sections on pages that have a side
nav.

app/assets/stylesheets/govuk-frontend/overrides.scss

@@ -18,3 +18,13 @@
 .govuk-main-wrapper {
   position: relative;
 }
+
+// Additional padding-bottom override, following the GOV.UK Frontend spacing scale:
+// https://design-system.service.gov.uk/styles/spacing/#the-responsive-spacing-scale
+.govuk-\!-padding-bottom-12 {
+  padding-bottom: 70px;
+
+  @include govuk-media-query($from: tablet) {
+    padding-bottom: 90px;
+  }
+}