Add a breadcrumb to the product page

This copies what the other GaaP components will be doing for their
product pages.

The SASS and HTML is taken from here:
f05ca1fb71/source/stylesheets/modules/_breadcrumbs.scss

Only changes I’ve made are:
- making the file paths work with our build pipeline

Changes to our code to accomodate this are:
- putting the padding on the product page `<h1>` not its container
- moving the hero image accordingly so that it lines up
- making the `<main>` element on the product page into an anchor so that
  the breadcrumb can link to it – screenreader will then announce the
  link as “GOV.UK Notify, same page”

app/templates/fullwidth_template.html

@@ -1,7 +1,7 @@
 {% extends "admin_template.html" %}
 
 {% block fullwidth_content %}
-  <main role="main">
+  <main role="main" id="main">
     {% block maincolumn_content %}{% endblock %}
   </main>
 {% endblock %}