Edges and Nodes: Model Refactor

[stuartc]

Details

See: #793

• Create a new WorkflowEdge model.
• Create a service to create a Workflow that creates the WorkflowEdges,
  Jobs and Triggers.
• Hunt down the places (starting with the test fixtures) where we create a
  Workflow, Job and Trigger and replace them with the new service.
• We may need to relax the database constraints on the Trigger model to allow
  for upstream_job_id to be blank.
• Triggers no longer require on_success and on_failure types.

Implementation notes