Add logic for respecting browser DNT setting

app/assets/javascripts/tracking.js

@@ -15,8 +15,14 @@ const extractData = (el, opts = {}) => {
 };
 
 export default class Tracking {
+  static trackable() {
+    return !['1', 'yes'].includes(
+      window.doNotTrack || navigator.doNotTrack || navigator.msDoNotTrack,
+    );
+  }
+
   static enabled() {
-    return typeof window.snowplow === 'function';
+    return typeof window.snowplow === 'function' && this.trackable();
   }
 
   static event(category = document.body.dataset.page, event = 'generic', data = {}) {

spec/frontend/tracking_spec.js

@@ -15,6 +15,12 @@ describe('Tracking', () => {
       snowplowSpy = jest.spyOn(window, 'snowplow');
     });
 
+    afterEach(() => {
+      window.doNotTrack = undefined;
+      navigator.doNotTrack = undefined;
+      navigator.msDoNotTrack = undefined;
+    });
+
     it('tracks to snowplow (our current tracking system)', () => {
       Tracking.event('_category_', '_eventName_', { label: '_label_' });
 
@@ -31,6 +37,27 @@ describe('Tracking', () => {
 
       expect(snowplowSpy).not.toHaveBeenCalled();
     });
+
+    it('skips tracking if the user does not want to be tracked (general spec)', () => {
+      window.doNotTrack = '1';
+      Tracking.event('_category_', '_eventName_');
+
+      expect(snowplowSpy).not.toHaveBeenCalled();
+    });
+
+    it('skips tracking if the user does not want to be tracked (firefox legacy)', () => {
+      navigator.doNotTrack = 'yes';
+      Tracking.event('_category_', '_eventName_');
+
+      expect(snowplowSpy).not.toHaveBeenCalled();
+    });
+
+    it('skips tracking if the user does not want to be tracked (IE legacy)', () => {
+      navigator.msDoNotTrack = '1';
+      Tracking.event('_category_', '_eventName_');
+
+      expect(snowplowSpy).not.toHaveBeenCalled();
+    });
   });
 
   describe('tracking interface events', () => {