This adds a field `supervisor_id` that aims to replace the `create_uid`
field. To prevent regressions, the old `create_uid` field is set to be a
computed field that has the same value as `supervisor_id`. This allow to
ensure that other part of the program that refer to `create_uid` as the
responsible for a purchase order will get the value of `supervisor_id`.
This new version comes with a migration script that fill the new
`supervisor_id` with the value of `create_uid` if `supervisor_id` is not
set yet.
This trick is done because the `create_uid` field is a magic field that
belongs to the ORM. This field cannot be modified in a form.
Rémy Taymans