Delay the initial AJAX call by 2 seconds

At the moment the first AJAX call is triggered as soon as the page
loads. We then look at its response time to work out how long to wait
until making the next call.

This isn’t great because:
- stuff is unlikely to have changed straight away, so it’s a waste of a
  call
- while the DOM is being updated it seems to introduce a delay in
  clicks on links, which is either more pronounced or more noticeable
  when it’s happening straight away, making the UI feel less snappy

I chose a value of 2 seconds as a rough proxy for the minimum time we’d
expect to see a notification go from created to delivered. Median
time-to-delivered was 2.9 seconds when we analysed it for
https://github.com/alphagov/notifications-admin/pull/2974#discussion_r286101286

app/assets/javascripts/updateContent.js

@@ -3,6 +3,7 @@
 
   var queues = {};
   var dd = new global.diffDOM();
+  var defaultInterval = 2000;
   var interval = 0;
 
   var calculateBackoff = responseTime => parseInt(Math.max(
@@ -54,11 +55,14 @@
 
   global.GOVUK.Modules.UpdateContent = function() {
 
-    this.start = component => poll(
-      getRenderer($(component)),
-      $(component).data('resource'),
-      getQueue($(component).data('resource')),
-      $(component).data('form')
+    this.start = component => setTimeout(
+      () => poll(
+        getRenderer($(component)),
+        $(component).data('resource'),
+        getQueue($(component).data('resource')),
+        $(component).data('form')
+      ),
+      defaultInterval
     );
 
   };