[manual] send_message: Rename recipient/stream fields to 'to'.

This commit changes APIs and requires and update of all zephyr
mirroring bots to deploy properly.

(imported from commit 2672d2d07269379f7a865644aaeb6796d54183e1)

examples/send-message

@@ -40,6 +40,6 @@ client = api.common.HumbugAPI(email=options.sender,
 message_data = {
     "type": "private",
     "content": "test",
-    "recipient": args[0]
+    "to": args[0],
 }
 print client.send_message(message_data)