When I use two clients (Desktop and phone), I want my phone to reset the "unread" counters on messages that I read and answered to from the phone.

When Monal receives a 'sent' carbon (not a 'received' one!), it should:

• remove the notification of the respective chat
• reset the message counter for that chat to 0
• reduce the message counter badge on Monal

In the future, it would also great to silence/delay Monal/mobile notifications if there was recent activity on the Desktop, cf. yaxim-org/yaxim#114 (comment)