[FIX] website_forum: fix forum back button url from backend

Currently, when user open the question from helpdesk ticket and
user clicks on the back button of the forum page,
it redirects the user to the app switcher. It happens because the
back button contains the homepage URL as httprequest.referrer
does not support the fragment url with hash(#).

so this commit fixes the issue by binding the onclick event on the
back button so clicking on the back button redirects the user
to the helpdesk ticket.

task-2602604

closes odoo/odoo#75585

Signed-off-by: Laurent Stukkens (ltu) <ltu@odoo.com>