refactor(case): Refactor case cost model.

[metroid-samus]

Previously case cost response calculation involved the following steps:

1. Retrieve: most recent recorded case response cost.
2. Aggregate all new participant response time since the last time we updated that response cost
3. Add the results from steps 1 and 2
4. Write over the recorded incident response cost by multiplying the result from step 3 with the employee hourly rate

This PR updates the case cost calculation by recalculating the case response cost from beginning to end based on all logged participant activities.

This reduces accumulated rounding errors.