In helpdesk_mgmt v16, ``_compute_user_id`` automatically filled
``user_id`` with ``team_id.alias_user_id`` when the team was set and
no user was assigned. This behaviour was removed in v18: the compute
now only clears ``user_id`` when the assigned user is not a member of
the selected team, without assigning anyone in its place.
This commit restores equivalent behaviour using ``team_id.user_id``
(the Team Leader field) instead of ``alias_user_id``:
- When ``team_id`` changes and the current ``user_id`` is not a member
of the new team, the Team Leader is assigned automatically.
- If the team has no leader configured, ``user_id`` is left unchanged
to avoid creating unassigned tickets.
- ``super()`` is not called: this is a full replacement of the v18
OCA behaviour, not an extension of it.
Also updates README.rst to document the new auto-assignment behaviour.
