Clear the notification when the message is read elsewhere

Since iOS 13 and the notification extension service, it is harder to make the app synchronise in the background with the server, which is the best to respect user privacy.
This makes the support of this feature harder.

For the moment, notifications are cleared only when they have been all read. In this case, the server send a badge count of 0 and we can clear all of them.

We need to find a way to resurrect this feature post iOS13.