This differential extracts a common logic that uses thread subscription, user role and mentioning to decide whether to send a notif. Moreover this logic is used in the client-send code.

1. Test that chat muting and user mentioning (also for ENS users) works on the keyserver from the notif perspective.
2. To test the client, apply patch from D12477 and then:
   1. Without any changes there should be no notif payloads generated.
   2. Play around with hardcoding different configurations of thread subscriptions and examine content of generated notifs.