Store joining_project in user_details.onboarding_status

Add form only object to user_details.onboarding_status as user_details.onboarding_status_joining_project

• Add new field to app/validators/json_schemas/user_detail_onboarding_status.json code that uses it/fills it first in separate MR. See #461404 (closed) - !173731 (merged)
• Update forms to allow to be saved on the user object - !173744 (merged)