fix bug 859875 - email change notifications UI

Pre-req's:
1. Make sure you've run `vagrant provision` to add the django tidings tables via schematic
2. Make sure you're running `manage.py celeryd` to send the email via celery

Spot-check:
1. Add 'page_watch' waffle flag set to super-users
2. Verify only super-users see the new button on the document page
3. Watch a page
4. As another user, make an edit to the page
5. Check the django console for the email output.