[ADD] mail: abstract thread model

Thread model that group all kinds of threads, including frontend threads
(e.g. website livechat).

addons/mail/static/src/js/models/threads/abstract_thread.js

@@ -0,0 +1,22 @@
+odoo.define('mail.model.AbstractThread', function (require) {
+"use strict";
+
+var Class = require('web.Class');
+
+/**
+ * Abstract thread is the super class of all threads, either backend threads
+ * (which are compatible with mail service) or website livechats.
+ */
+var AbstractThread = Class.extend({
+    init: function (parent, data) {},
+
+    //--------------------------------------------------------------------------
+    // Public
+    //--------------------------------------------------------------------------
+
+    getMessages: function () {},
+});
+
+return AbstractThread;
+
+});