[FIX] im_livechat: use the currentPartnerId from livechat

Current code can't work since session doesn't contain partner_id.

task-3010650

closes odoo/odoo#102952

X-original-commit: dcf9aa612e76ea71429725e12a819678c09cf449
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/im_livechat/static/src/legacy/models/public_livechat_message.js

@@ -260,7 +260,7 @@ const PublicLivechatMessage = Class.extend({
      * @return {boolean}
      */
     _isMyselfAuthor() {
-        return this.hasAuthor() && (this.getAuthorID() === session.partner_id);
+        return this.hasAuthor() && (this.getAuthorID() === this.messaging.publicLivechatGlobal.options.current_partner_id);
     },
 
 });