[IMP] Added `hierarchy_level` and relations control to `res.company`

@enricostn @xavier.bonet We can assure that base.main_company will be always the instance company? This is the logic added in the post init hook.

Features

• New hierarchy_level selector field in res.company
• Constrains on hierarchy_level
  • Can only be one instance company
  • instance company can't have any parent_id company
  • coordinator parent hierarchy_level must be instance
  • community parent hierarchy_level must be coordinator
• Added parent_id_filtered_ids computed field to create dynamic domain to parent_id depending on the hierarchy_level
• Added parent_id field to easy_company_creation (Domain is not dynamic, we only create community type here)
• Added unit test. (Feedback is appreciated 😅)
• Post init hook assigns instance to base.company and the children company coordinator

Roadmap

• Enable creation of coordinator companies in easy company creation