Sync notifications

[Thithic]

Description:

I use matrix clients on my laptop and my phone. When I receive a message, notification is sent to both devices.

If I read the message on my laptop, the notification on my phone remains, although the messages are now read.
Since ntfy supports deleting notifications as of version 2.16 (see https://docs.ntfy.sh/releases/#ntfy-server-v2160), it should be possible for Synapse server to send a notification deletion when a message is read.