Migrate from using default case cost type to assigning cost type by cost model.

[metroid-samus]

Prerequisite for implementing parallel cost model execution for cases, allowing both the new cost model and classic cost model to run simultaneously and maintain separate cost records.

In this PR, we:

• replace the cost_type default column with a cost_model_type column:
  • model_type=new is the response cost type associated with the new cost model
  • model_type=classic is the response cost type associated with the classic cost model
  • model_type=None is a cost type that is not associated with response time. This would cover manually-added costs accumulated over the course of the incident.
• Update scheduler code to create a case response cost type and case response costs if none exist.
• Update the UI to indicate which cost types are tied to response costs.