Fetch template length message as user types

This commit adds some Javascript that makes AJAX requests as the users
changes the content of their template.

It then takes the content returned by the backend and inserts it in the
page.

app/assets/javascripts/updateStatus.js

@@ -0,0 +1,41 @@
+(function(window) {
+  "use strict";
+
+  window.GOVUK.Modules.UpdateStatus = function() {
+
+    let getRenderer = $component => response => $component.html(
+      response.html
+    );
+
+    this.start = component => {
+
+      let id = 'update-status';
+
+      this.$component = $(component);
+      this.$textbox = $('#' + this.$component.data('target'));
+
+      this.$textbox
+        .on('input', this.update)
+        .trigger('input');
+
+    };
+
+    this.update = () => {
+
+      $.ajax(
+        this.$component.data('updates-url'),
+        {
+          'method': 'post',
+          'data': this.$textbox.parents('form').serialize()
+        }
+      ).done(
+        getRenderer(this.$component)
+      ).fail(
+        () => {}
+      );
+
+    };
+
+  };
+
+})(window);