Moved the User.subscribe and User.unsubscribe methods to the UserSubscriptionHelpers module.

This way User model is simplified, and methods related to subscribing and unsubscribing are together in their own module.
Moved methods related with refreshing feeds and folders from the User model to a new module, UserRefreshHelpers.

This continues the line of simplification of the User model, which had become bloated, by moving functions to separate modules which are included in the User model. Modules group methods with related functionality.