Resolve duplicate FAQ schemas

We show the whole of a guide's FAQ schema on each of its chapters. I think
this is OK, as long as the URLs that refer to the questions and answers are
correct. (We might change this at some point though)

The FAQ schema was appending the part slugs to the canonical URL of the page
that it was displayed on.  This works well for the default page in a guide,
but less well for other chapters in the guide.

The canonical URL is set (correctly) to [the URL of the chapter](https://tools.ietf.org/html/rfc6596).
This means we were generating incorrect URLs.

Correct example when viewing default page in a guide:

When you view https://www.gov.uk/christmas-bonus you get links in the structured
data like:
- https://www.gov.uk/christmas-bonus/eligibility
- https://www.gov.uk/christmas-bonus/how-to-claim

Incorrect example when viewing sub chapter in a guide:

When you view https://www.gov.uk/christmas-bonus/eligibility
you get links in the structured data like:
- https://www.gov.uk/christmas-bonus/eligibility/eligibility
- https://www.gov.uk/christmas-bonus/eligibility/how-to-claim

This fix should get us links like:
- https://www.gov.uk/christmas-bonus/eligibility
- https://www.gov.uk/christmas-bonus/how-to-claim

lib/govuk_publishing_components/presenters/machine_readable/faq_page_schema.rb

@@ -42,11 +42,15 @@ def questions_and_answers
 
       def part_url(part, index)
         if index.zero?
-          page.canonical_url
+          guide_url
         else
-          page.canonical_url + "/" + part["slug"]
+          guide_url + "/" + part["slug"]
         end
       end
+
+      def guide_url
+        Plek.new.website_root + page.base_path
+      end
     end
   end
 end

spec/lib/govuk_publishing_components/presenters/schema_org_spec.rb

@@ -63,7 +63,10 @@
     end
 
     it "generates schema.org FAQPages" do
-      content_item = GovukSchemas::RandomExample.for_schema(frontend_schema: "guide") do |random_item|
+      content_item = GovukSchemas::RandomExample.for_schema(
+        frontend_schema: "guide",
+        canonical_url: "http://www.dev.gov.uk/how-to-train-your-dragon/insurance"
+      ) do |random_item|
         random_item.merge(
           "base_path" => "/how-to-train-your-dragon",
           "details" => {