feat(preferences) - change the way user preferences are registered.

Follow-up for https://github.com/superdesk/superdesk-server/pull/276
yep for the current user we can do this. but we will need to get the preferences not only for current user on the backend. so eg sending an email on notification. current user writes a comment. for all the mentioned users you will want to check their preferences and send an email if it's set to true. so probably it should be like `get_user_preference(preference, user)`