Move and adapt gantt unavailability related methods ( _get_work_interval() and _get_unavailable_intervals() ) from hr.employee to resource.resource. This move allows the unavailability to be generated for users without related employees (like in FSM in enterprise). Task-2069861 closes odoo/odoo#36785 Signed-off-by: Jérome Maes (jem) <jem@openerp.com>