[FIX] im_livechat: show message history

Revision on https://github.com/odoo/odoo/commit/923fb7aa05f0e9e0e67440073993ec36bb0f5f53

Commit above fixed an issue since v11.0 that display duplicated
messages on a website livechat window.

Forward-ported broke the livechat in v12.0 due to refactoring of mail
https://github.com/odoo/odoo/commit/cd34f6de727d5b3858420cff3c9d3c5995c4e75c

This commit correctly adapts fix for v12.0+.

opw-2070911

closes odoo/odoo#36921

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/im_livechat/static/src/js/im_livechat.js

@@ -125,7 +125,7 @@ var LivechatButton = Widget.extend({
         var message = new WebsiteLivechatMessage(this, data, options);
 
         var hasAlreadyMessage = _.some(this._messages, function (msg) {
-            return message.id === msg.id;
+            return message.getID() === msg.getID();
         });
         if (hasAlreadyMessage) {
             return;