[FIX] website_slides: allow portal users to share slides by email

Sharing content by email as portal is not working and raises an AccessRecord
because we are trying to access the "email_formatted" field of the company.

This is solved by adding 'compute_sudo' to the computed field.

Indeed, we want portal users to be able to read the email address of the
company while they cannot access res.partner records.

A tour has been adapted to ensure this behavior.

Task-3349132

closes odoo/odoo#123647

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