A bit of DRYing.

The work of updating the date/time of the last change in subscriptions is done in FeedSubscription#touch_subscriptions.

This method:

- touches the FeedSubscription instance, setting its updated_at attribute to the current datetime
- sets the subscriptions_updated_at attribute of the associated user to the current datetime
- if the feed is in a folder owned by the user, sets the subscriptions_updated_at attribute of the folder to the current datetime.

This method is invoked from multiple places:

- when creating a subscription
- when deleting a subscription
- when moving a feed into a folder
- when moving a feed out of a folder
- when the unread entries count of a feed changes
- when the title of a feed changes
- when the url of a feed changes