* Add "responsible_owner" field to part model
- Will replace "responsible" field
* Data migration
- Adds 'responsible_owner' value for parts which have 'responsible' set
- Selects correct content type
- Performs reverse migratoin
* Update part serializer
- Point to the new field
- Rename to preserve compatibility
- OPTIONS metadata will take care of the rest
* Remove old 'responsible' field
* Bump API version
* Fix typo
* Fix serializer field