Fine-grained notifications on forum posts.

Description

Right now, we have an opt in for email notifications on forum post answer which is great. We do not have a possibility to switch the behaviour on bell notices, though. Maybe this could be adapted.

The number of bells is quite high right now. We should just notify for the first answer until the thread has been visited.

Impact

Foodsharers

Proposal

1. add the possibility to opt out from bell notices.
2. Show bell and/or mail for the first answer only until the FS has visited that thread again.

Links / references

discussion: https://foodsharing.de/?page=bezirk&bid=734&sub=forum&tid=62529

Discussion including screenshots in Slack: https://yunity.slack.com/archives/C74JC2V26/p1581581429147800