[FIX] im_livechat: 'Visitor' translated in livechat

Make translation work for "Visitor" that was appearing in livechat
session to the visitor.

opw-2504461

closes odoo/odoo#70139

X-original-commit: 738ef4ce8040abc7c11a578b6208162d39a686f7
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>