[IMP] project: substask can live their own life

This commit completes what was initiated a few time ago : a substask
is like a normal task and can now live its own life, being in another
project, attached to another customer, billed on another sale line, ...
than its mother.
The informations coming from the parent are only use as default values to
prefill the form view (with default options on creation, or though `onchange`
when setting the parent on existing task).

No values are forced now.

Form, default context, and others views has been adapted to make this feature
works.

Task-1982072

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