Create Vulnerabilities::StateTransition model

Why are we doing this work?

We need to introduce VulnerabilityStateTransition model which will be responsible for storing state transitions of a Vulnerability. This will allow us to:

• Deprecate and remove Vulnerabilities::Feedback model
• Drop vulnerability_feedbacks table
• Add comments to all state transitions
• Not immediately on the horizon but we would be able to quickly build an audit trail for Vulnerability state changes

See #292636 (comment 516669807) for details why the current architecture is not good enough.

Implementation plan

• database Create a migration to create vulnerability_state_transitions with
  • id:primary_key
  • from_state:integer - reflecting EE::Vulnerability.state enum
  • to_state:integer - reflecting EE::Vulnerability.state enum
  • vulnerability_id:references
• backend Create Vulnerabilities::StateTransition model