Time completion: time elapsed to answer a proposal

[microstudi]

Original request

As a process-admin, I would like to see the time of completion in the Excel export (as soon as the status changes to completed/accepted).

Context

Proposals have a answer state and a answered_at time field. In the admin it is possible to see the state but not values on how long it took to answer the proposals (and giving a final state).

Tasks

• Add the time elapsed information in a new method in the proposals model
• Use this info for answered proposals, show it in the admin?
• Add this info in a column in the generated CSV/Excel when exporting proposals
• Tests