Received notifications without a server are silently ignored, should be highlighted

If the app receives a notification, it will try to match the webhook ID in the payload to a server and store the received data + matched server ID. Depending on whether a server is found, it may be ignored (not posted), invisible to the user.

To make this issue more visible, the notification history should highlight notifications without a matching server ID. Might be good to pick up as part of a larger redesign of the list, and needs some discussion before implementation.