Make spacing between breadcrumbs and heading consistent

Make the spacing on single page layouts consistent with the spacing
on the side page pages.

This includes moving the `app-width-container` outside of `<main>`
so that the horizontal spacing is more easily applied to both
the breadcrumbs and `<main>`

Author: romaricpascal

src/stylesheets/main.scss

@@ -78,8 +78,7 @@ $app-code-color: #d13118;
 .app-breadcrumbs {
   // match top padding of the first item in the pane navigation so that the
   // breadcrumbs line up nicely
-  margin-top: govuk-spacing(1);
-  margin-bottom: 0;
+  margin: govuk-spacing(1) 0;
 }
 
 .app-content {

views/layouts/layout-single-page.njk

@@ -1,16 +1,15 @@
 {% extends "_generic.njk" %}
 
 {% block body %}
-  {{ govukBreadcrumbs({
-    classes: "app-width-container",
-    items: getAncestorPages(permalink)
-  }) }}
+  <div class="app-width-container">
+    {{ govukBreadcrumbs({
+      items: getAncestorPages(permalink)
+    }) }}
 
-  <main id="main-content" class="govuk-main-wrapper govuk-main-wrapper--l" role="main">
-    <div class="app-width-container">
+    <main id="main-content" role="main">
       {{ contents | safe }}
-    </div>
-  </main>
+    </main>
+  </div>
 
   {% include "_footer.njk" %}
 {% endblock %}