[FIX] sale_timesheet: SO line on subtask

- Activate sub-stasks on projects
- Create a product A:
  Product Type: Service
  Invoice based on: Timesheets on tasks
  Service Tracking: Create a task in an existing project
- Create a SO with A, validate
- In the project view, open the created task
- Click on the stat button 'Sub-tasks'
- Create a task from the kanban view

The SO line is not copied on the sub-task.

The `name_create` method doesn't send the `parent_id` key in the values.
Therefore, the SO line is not retrieved.

We fallback on the context key `default_parent_id`.

opw-1857560

addons/sale_timesheet/models/project.py

@@ -70,8 +70,9 @@ class ProjectTask(models.Model):
     @api.model
     def create(self, values):
         # sub task has the same so line than their parent
-        if 'parent_id' in values and values['parent_id']:
-            values['sale_line_id'] = self.env['project.task'].browse(values['parent_id']).sudo().sale_line_id.id
+        parent_id = values['parent_id'] if 'parent_id' in values else self.env.context.get('default_parent_id')
+        if parent_id:
+            values['sale_line_id'] = self.env['project.task'].browse(parent_id).sudo().sale_line_id.id
         return super(ProjectTask, self).create(values)
 
     @api.multi