[FIX] point_of_sale: fix strange mail construct

Commit c95acd62 added support of sending tickets to client directly
by email. However parameters management in the business method allows to send
void emails or to spam people quite easily. Moreover missing translations
marks prevent from translating mail content.

This commit fixes that by ensuring current user is member of pos user group.
Code is also a bit cleaned to ensure mail values are correct. Finally
translations marks are added to allow content translation.

closes odoo/odoo#39252

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