To-Do API should include updated_at timestamp

This issue is part of the broader theme, Embedded Analytics: To-Do & Personal Pages.

Problem to solve

The public API for retrieval of To-Do List items omits the update timestamp on to-do records, those it does include created date. Happily the update values are stored in the DB, just not exposed via the API. I was hoping maybe I could scrape that from an XHR made to an internal API, but no such luck. The page is rendering a server-side template and doesn't include that metadata.

Intended users

This may be useful to users generally but it will be especially handy to those doing research in support of the epic listed above as we will need the time at which items are closed for many important calculations. I had written a utility that consumed the API in order to collect my own data from production data for some experimentation but without the update timestamp, I am pretty well stuck.

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.