[IMP] project: drop project substask constraint

This commit improves subtask mecanism, on several points:
1) Drop the project constraint: before this commit, creating
subtask in non parent project was not possible. Now you can move
subtask to any project.
The default project of a subtask is the one from the parent.
2) When setting a parent task, we want to force some field to have
the same value as its parent, like `partner_id` or the `sale_line_id`.
The idea is here, we work for the same client; a subtask count for the
same goal (SO line or customer). Those fields should be readonly on the
view.
3) Refactor some methods

Task: 38498

0 files changed, 0 insertions, 0 deletions