Add `<no-store/>` hint on outgoing chat-state-notifications (CSNs)

poezio is sending CSNs with the following syntax:

<message type="chat" to="recipient" xml:lang="en" id="6594b9c6b6984e0e8d9fb8754323ecf7">
  <origin-id xmlns="urn:xmpp:sid:0" id="6594b9c6b6984e0e8d9fb8754323ecf7"/>
  <composing xmlns="http://jabber.org/protocol/chatstates"/>
</message>

CSNs should have the <no-store/> Hint to prevent temporary or permanent storage on the sender's or recipient's archive.