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/server/libs/inbox/index.js

@@ -0,0 +1,11 @@
+export async function deleteMessage (user, messageId) {
+  if (user.inbox.messages[messageId]) {
+    delete user.inbox.messages[messageId];
+    user.markModified(`inbox.messages.${messageId}`);
+    await user.save();
+  } else {
+    return false;
+  }
+
+  return true;
+}