unread counts: add support for threaded receipts

[jmartinesp]

The PR #3054 fixed a bug with implicit receipts where:

1. You receive a mention in a thread.
2. You reply to that thread in a different client (Element Web in my case).
3. You open the room in EX and the badge doesn't get cleared.

However, if we want to get the behaviour 100% right I believe the badge should be cleared as soon as the other client opens the thread (in fact, its own unread status is cleared in that client).

So the problem I see now is:

1. You receive a mention in a thread.
2. You open it in another client (EW?). The unread gets removed in this client.
3. The unread marker and the mention badge are still present in the EX client.

Opening the room in EX fixes the stuck unread and mention indicators, but it shouldn't be needed.

[bnjbvr]

Thanks for opening an issue. In other words, this would require support for threaded read receipts, which I don't think Element has on the short-term roadmap, so managing expectations here :-)