WIP: update updated_subscriptions_at attribute of User instances when appropiate.
WIP: update subscriptions_updated_at attribute in User instances at the appropiate times.
Use ActiveRecord callback to calculate the initial unread count when creating a feed subscription, instead of invoking a method as part of subscription.

It's nicer to be declarative rather than imperative. Also the new method is a bit faster because it just counts entries without looking at entry states (when a subscription is created, all entries in the feed are unread for that user).