Feature Request: allow to mark actions as completed

[armisael]

As a user, I want to be able to mark actions of past retrospectives as "completed", so that I can focus on what needs to be done, every week.

Acceptance Criteria:
The dashboard of old retrospectives allows to mark each action as "completed". Every user can do this, not only the assignee.

• Each action has a checkbox close to it
• The checkbox can be checked and unchecked by everyone
• The checkbox is stored in the backend

[vanderhoop]

that's a reasonable ask. let me think on the design aspects, as we'd need to improve the dashboard view's athetics and info surfacing. right now it's fairly bare bones