Move tracking code into a sepparate file

[MAILPOET-1539]

assets/js/src/nps_poll.jsx

@@ -0,0 +1,15 @@
+function displayPoll() {
+  if (window.display_nps_poll && window.satismeter) {
+    window.satismeter({
+      writeKey: '6L479eVPXk7pBn6S',
+      userId: window.current_wp_user.ID + window.site_url,
+      traits: {
+        name: window.current_wp_user.user_nicename,
+        email: window.current_wp_user.user_email,
+        createdAt: window.mailpoet_settings.installed_at,
+      },
+    });
+  }
+}
+
+setImmediate(displayPoll);