Allow for deleting exports

[maxkadel]

Create a rake task that will delete old exports

Acceptance criteria

• Has a configurable TTL for completed import directories (e.g., delete after 30 days if the associated exporter run has a terminal status: Complete or Failed)
• Ensure in-progress exports are not cleaned up
• Files are deleted
• Database records are deleted

Implementation notes

Should probably kick of a background job or jobs, rather than trying to do it in the terminal session