Notification Digester on deleted elements

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When sending periodic notifications (each hour, day or week) the process building the URLs needs to take into account which elements from the notification have been removed, so they need to be ignored and not included in the body of the email.

https://github.com/PopulateTools/gobierto/blob/master/app/decorators/user_notification_decorator.rb#L21

Rollbar: https://rollbar.com/fernando/gobierto/items/420/

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• cron task that removes user notifications with nil subject in the last 2 hours
• run the task every hour
• fix the code for real time deliveries