Issue and incident templates to generate todos

Context

issues templates and incident templates where using the gitlab quick action /todo, but I was requesting too much from it, and it cannot take as many parameters as expected. The best way to generate a todo for some other users than oneself is to mention that user at the start of the line : cf. https://docs.gitlab.com/ee/user/todos.html

This MR fixes this for reviews and validation in the incidents and change request of the umami project templates.