Allow enabling/disabling individual notification channels per app

[DanielJutz]

Is your feature request related to a problem?

I would like to disable the persistent notification for K9 mail which is in a separate channel, but not other notifications from K9 mail.

Description

Provide a checkbox for each notification channel per app to enable disable sending those notifications.

[patri9ck]

After a quick research, I don't think this is possible. For this to work, the app would need to know the channel IDs K-9 mail uses before any notification is ever posted which does not work.