-> Move Subtasks mechanism from hr_timesheet to project.
-> Various functional improvements:
- Add new 'Sub-tasks' setting and 'Sub-task Project' field based on
this setting.
- Relabel 'Team Collaboration' section to 'Task Management'.
- Remove 'Customer Satisfaction' section and move setting
'Rating on Tasks' into 'Task Management' section.
- Add customer name on the project's kanban view.
- Set default value of 'Sub-task Project' field is the same project.
- Add 'Parent Task' stat button on task form view.
- Make 'Sub-Tasks' stat button visible when no parent task