This allows notification content to be updated in the web app by sending multiple messages with the samesequence id (sid).

Here is a small video demo:

I tried to make sure that things still work if this new version of the web app subscribes to a topic on an "old" ntfy server (a server that doesn't providemtime orsid fields).

The server doesn't support "deleting" notifications (dismissing from the notification shade) yet, just updating.

The web app cannot "delete" notifications programmatically since "silent" pushes are not allowed. (i.e., sending a JSON object with a message ID to delete and not showing any notification as a result of that JSON object is not allowed. Deleting should work fine on Android, though.)

@binwiederhier you'll need to decide how we present all this visually in the web app. I just wanted to get the information on there somewhere.

All the revisions of a particular message are attached to the one visible notification, so we could create some sort of dialog/modal to show the past revisions of a message. But I figured we could do that later, once everything else is working.