Update event factory and features to reflect actual URL structure

The website listings feed contains the url for the event listing page,
not the buy tickets page - update features and event factory to avoid
confusion in test cases.
Add canonical links back to main site on all event pages

To avoid mobile site showing ahead of www.edbookfest.co.uk in google
results, add the URL for the event page on the main site as a canonical
url link in the header of each event detail page.
Add Google Site Verification code to homepage header

Set a google-site-verification meta header based on an environment variable
to allow management of site indexing with google webmaster tools.