[HR] clean model in a changes the hr_applicant model in add the _content...

..._serach method and hr_contract models in add the xml id on the serach and remove _resolve_department_id_from_context method in hr_recruitment model.and add a method _broadcast_welcome hr_recuriemnt model.

- Task: https://www.odoo.com/web#id=8871&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.f9a1kXos2Rn0aS5d
Seems curious to have a related on a one2many field. I think what was asked in the task is to have a filter that search on attachment_ids.index_content; having a related on a 2many will lead to some strange results.