Add Google Analytics

https://www.pivotaltracker.com/story/show/115861923

Makes some changes to the content security policy, to allow
the Google Analytics JS and trakcing image to be loaded,
copied from @alexmuller’s excellent work on GOV.UK:

https://gdstechnology.blog.gov.uk/2015/02/12/experimenting-with-content-security-policy-on-gov-uk/
https://github.com/alphagov/frontend/pull/733

app/templates/admin_template.html

@@ -83,4 +83,12 @@
 
 {% block body_end %}
   <script type="text/javascript" src="{{ asset_url('javascripts/all.js') }}" /></script>
+  <script>
+    (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
+      (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
+      m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
+    })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
+    ga('create', 'UA-75215134-1', 'auto');
+    ga('send', 'pageview');
+  </script>
 {% endblock %}