[FIX] survey: fix invite mail template
[FIX] survey: fix invite mail template

Purpose of this commit is to improve the recently-introduced mail template
used to invite people to surveys. Containing

 * fix the display of deadline that is not required;
 * correctly talk about survey, not apparaisal;

This commit is linked to task ID 1932508 (side-dish fix).

closes odoo/odoo#30537
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo