Refactor CallRequest Status Handling

[jesalg]

• Add a new foreign key column called 'actor_id' to call_requests.
• Reduce the call_requests statuses back to:
  • Proposed
  • Approved (Remove ApprovedMember & ApprovedMentor)
  • Changed (Remove ChangedMember & ChangedMentor)
  • Declined (Remove DeclinedMember & DeclinedMentor)
  • Cancelled (Remove CancelledMember & CancelledMentor)
  • Completed
  • Processed
• Update call_request API to accept 'actor_id' along with the 'call_request_status'. So we know who changed the status.