Make it possible to declare in the create task operation email-templates that will be used for notifications around the task

There should be three emails templates: one for task assignment, one for task acceptance and one for task rejection. In those emails, we should be able to use : the task creator first name, last name and id, the date of the task, the directive, the comment of the task, the first name, last name and id of the user that accepted or rejected the task.