Migration of the modules timesheet_task and hr_timesheet_task to 8.0

This pull request port the modules hr_timesheet_task and timesheet_task to v8.

There is a regression: the model ‘project.task’ does not include a fixed list of state anymore, but a dynamic list where the user is free to add, change and delete any states he want. The drawback is that it is not possible to exclude any closed task when selecting a task from a drop down list.