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/templates/fullwidth_template.html

@@ -1,5 +1,7 @@
 {% extends "admin_template.html" %}
 
+{% set mainClasses = "govuk-!-padding-top-0 govuk-!-padding-bottom-0" %}
+
 {% block main %}
   {% block beforeContent %}{% endblock %}
   <main class="govuk-main-wrapper {{ mainClasses }}" id="main-content" role="main">