If leads are created using live chat, users without CRM rights cannot
access the visitors list anymore because the lead/opportunity
information is not available to them.

This commit limits the display of `lead_count` to the
`sales_team.group_sale_salesman` group.

In 14.0, the column still appeared without the `lead_count` value
displayed, but clicking on it raised a traceback.

Steps to reproduce:
- Install `website_crm_livechat`
- Login as Mitchell Admin
- Send a message in the live chat
- Go to Discuss
- Answer the livechat message with `/lead New`
- Go to Settings / Users / Marc Demo
- Remove the Sales rights
- Logout
- Login as Marc Demo
- Go to the Website / Reporting / Visitors page

=> The page could not be reached and an access right error message was
generated.

opw-3475301

closes odoo/odoo#133581

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>