Merge branch 'csn-no-store' into 'master'

Fix #3518: Add no-store hint to CSN

Closes #3518

See merge request poezio/poezio!88

poezio/tabs/basetabs.py

@@ -697,6 +697,7 @@ class ChatTab(Tab):
                 msg['type'] = self.message_type
                 msg['chat_state'] = state
                 self.chat_state = state
+                msg['no-store'] = True
                 msg.send()
                 return True