Display who took the last action on each response and when, in the UI response table [key actions]

## Context

As part of #6224, for accountability on teams where multiple people have access to form responses, we want users in GC Forms to be able to identify who downloaded/confirmed responses.

## Current state

## Proposed option

If it's possible / feasible for downloaded responses and signed off responses:
- Add column that includes the date and time of the last action taken
- Add column that includes the email address of the user who took the last action
- Update column "date" to be "submission date"

Alternatively we might offer users to download a log/history of responses with the activity log that Sam is working on.