This commit does some preparation work to allow the appointment type to specify a custom mail template to send when an appointment is booked/canceled. It simply adds a hook method to be able to customize the behavior when sending the invitation email as well as removes some parts of the current invitation template that will be rendered useless. See enterprise counterpart for more details. Task-3547269 closes odoo/odoo#138547 Related: odoo/enterprise#48874 Related: odoo/upgrade#5269 Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>