Add consent tracking to cookie functions

Taken from GOVUK components:

https://github.com/alphagov/govuk_publishing_components/blob/master/app/assets/javascripts/govuk_publishing_components/lib/cookie-functions.js

Also includes:
- make new cookie functions handle notify domains
- addition of hasConsentFor function to allow easy
  checking of consent for categories of cookie

app/assets/javascripts/consent.js

@@ -0,0 +1,15 @@
+(function (window) {
+  "use strict";
+
+  function hasConsentFor (cookieCategory) {
+    const consentCookie = window.GOVUK.getConsentCookie();
+
+    if (consentCookie === null) { return false; }
+
+    if (!(cookieCategory in consentCookie)) { return false; }
+
+    return consentCookie[cookieCategory];
+  }
+
+  window.GOVUK.hasConsentFor = hasConsentFor;
+})(window);