[16.0][IMP]helpdesk_mgmt: add 'Created Ticket' email template

[PicchiSeba]

New email template for newly created tickets. This email template is modelled like the already existing ones for Closed and Changed State)

This template was originally introduced in 98e5ef7f4a2e074283da3674654f662a28f02eed. As requested in https://github.com/OCA/helpdesk/pull/644#discussion_r1858034832 it has been moved into its own PR