Add support to get notification emails only for favorite questions/categories

Please review the changes (mostly 1f23b51) and if you are OK with them please pull or ask me to make this a separate plugin since it has significant changes from the original one.

The new design does not allow non logged in users to subscribe. It also does not allow a single user to subscribe with multiple e-mails.

The major contribution of this design is that a user can subscribe for notification e-mails only for his favorite questions or/and categories.