Make notification intervals configurable

[nuest]

Users should be notified about new papers for their subscriptions in an interval of their choice. The notification intervals are predefined:

• weekly
• monthly

The scheduling should collect all subscriptions at the beginning of each interval and send new all items that were added in the previous interval, so in the night from Sunday to Monday, all articles added between the previous Monday and that Sunday should be included in the notification.

If there are no new articles, no notification is sent.