Followers of topics

When a user favourites a topic they get added to a list of followers of
that topic.

This will let the notification system find who to drop a notification
too when something new is published.
No caching
Merge branch 'master' of github.com:Financial-Times/next-user-preferences-api into notifications

Conflicts:
	app.js
Merge pull request #17 from Financial-Times/notifications

Followers of topics