Link cookie consent "learn more" to privacy policy

To enable, create `privacy.md` in `content/` and add your privacy
policy.

Close #537

exampleSite/content/privacy.md

@@ -0,0 +1,12 @@
++++
+title = "Privacy Policy"
+
+date = 2018-06-28T00:00:00
+draft = false
+
+# [header]
+# image = ""
+# caption = ""
++++
+
+...

layouts/partials/cookie_consent.html

@@ -19,8 +19,9 @@
       "content": {
         "message": {{ i18n "cookie_message" | default "This website uses cookies to ensure you get the best experience on our website." }},
         "dismiss": {{ i18n "cookie_dismiss" | default "Got it!" }},
-        "link": {{ i18n "cookie_learn" | default "Learn more" }}
+        "link": {{ i18n "cookie_learn" | default "Learn more" }},
+        "href": {{ with .Site.GetPage "page" "privacy.md" }}{{ printf "%s" .Permalink }}{{ else }}"https://cookies.insites.com"{{ end }}
       }
     })});
 </script>
-{{ end }}
+{{ end }}