Option to only notify on new items

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## Feature request

**Which Nextcloud Version are you currently using:** Nextcloud Hub 8 (29.0.5)

**Is your feature request related to a problem? Please describe.**
Currently, enabling notifications on this app means that every time it checks I will always get a message saying I have thousands of unread items. I'm not going to get through all these now or anytime soon, and I don't want to be notified about my backlog repeatedly.

**Describe the solution you'd like**
I would like the option to be notified ONLY when something *new* is published in one of my feeds (that's primarily why I subscribe to them), and be told what it is.

**Describe alternatives you've considered**
Disabling notifications and checking the app every few days. This is what I do at the moment.