Update content area blocks in descendent pages

GOV.UK Frontend's template wraps all content in a
`<main>` tag and puts it in the main column.

Some of Notify's pages need to reach out of the
main column (product page).

Some have several blocks of navigation before the
main content area.

This adjusts the blocks in all the templates
where this is the case so those conditions can be
met.

app/templates/withnav_template.html

@@ -4,8 +4,8 @@
   {% block service_page_title %}{% endblock %} – {{ current_service.name }}
 {% endblock %}
 
-{% block fullwidth_content %}
-  <div id="content">
+{% block main %}
+  <div class="govuk-width-container">
     <div class="navigation-service">
       {% if current_service.organisation_id %}
         {% if current_user.platform_admin or
@@ -27,13 +27,18 @@
         {% include "main_nav.html" %}
       </div>
       {% if help %}
-        <main role="main" class="column-two-thirds column-main">
+        <div class="column-two-thirds column-main">
       {% else %}
-        <main role="main" class="column-three-quarters column-main">
+        <div class="column-three-quarters column-main">
       {% endif %}
-        {% include 'flash_messages.html' %}
-        {% block maincolumn_content %}{% endblock %}
-      </main>
+        {% block beforeContent %}{% endblock %}
+        <main class="govuk-main-wrapper column-main {{ mainClasses }}" id="main-content" role="main">
+        {% block content %}
+          {% include 'flash_messages.html' %}
+          {% block maincolumn_content %}{% endblock %}
+        {% endblock %}
+        </main>
+      </div>
     </div>
   </div>
 {% endblock %}