To-Do: Add button to "Close all"

Problem to solve

In order to be able to compute completion rate for To-Do items (as part of the larger capability described in Inbox Analytics) we need to be able to distinguish between a To-Do which was completed by executing which ever action was requested by the To-Do (such as responding to a comment in which the user was directly addressed, completing an approval, etc.) and a To-Do which was dismissed, meaning it was ignored because there were simply too many open To-Dos for the user to ever complete.

Intended users & other context

For more context about intended users and other details, please see the epic.

Proposal

• Todo has a new state called closed.
• On the Todos list page, there is a new button to "Close all" (to the right of the button to "Mark all as done").
• Clicking "Close all" has similar behavior to "Mark all as done". (If resolution_action has been introduced, the Todo has a resolution action value of close-all.)
• After clicking "Close all" it turns into "Undo close all" and has similar behavior to "Undo mark all as done".

Is this a cross-stage feature?

To-Dos are the domain of groupproject management overseen by @gweaver with whom we will confer as we work in this area.