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/assets/stylesheets/main.scss

@@ -59,6 +59,7 @@ $path: '/static/images/';
 @import 'components/tick-cross';
 @import 'components/list-entry';
 @import 'components/letter';
+@import 'components/vendor/breadcrumbs';
 
 @import 'views/job';
 @import 'views/edit-template';