New inbox client (#10644)

* new inbox client

* add tests for sendPrivateMessage returning the message

* update DELETE user message tests

* port v3 GET-inbox_messages

* use v4 delete message route

* sendPrivateMessage: return sent message

* fix

website/common/errors/apiErrorMessages.js

@@ -22,4 +22,6 @@ module.exports = {
   missingCustomerId: 'Missing "req.query.customerId"',
   missingPaypalBlock: 'Missing "req.session.paypalBlock"',
   missingSubKey: 'Missing "req.query.sub"',
+
+  messageIdRequired: '\"messageId\" must be a valid UUID.",',
 };