Design: notification settings

We should think about how we want to approach notifications in the future.
Right now the system is pretty simple - we only send emails, we send them in very specific cases and there's no way to opt out.
Since we want to extend notifications we should probably think about some generalized approach which would allow admins/users to disable select notifications, opt out from receiving them etc.

For example we want to add notifications on gateway disconnect (https://github.com/orgs/DefGuard/projects/5/views/2?pane=issue&itemId=35497548) which is obviously needed, but we also have a US East gateway, which reconnects every 30 seconds. It would be nice to be able to mute notifications for this GW.