Use canonical URL in opengraph tags instead of web URL

This ensures that a content item with parts will have the correct
URL.

Author: thomasleese

app/views/layouts/application.html.erb

@@ -29,7 +29,7 @@
 
   <meta property="og:site_name" content="GOV.UK" />
   <meta property="og:type" content="article" />
-  <meta property="og:url" content="<%= @content_item.web_url %>" />
+  <meta property="og:url" content="<%= @content_item.canonical_url %>" />
   <meta property="og:title" content="<%= @content_item.page_title %>" />
   <meta property="og:description" content="<%= strip_tags(@content_item.description) %>" />