fix(classifier): set started_at timestamp when annotation starts

[eatyourgreens]

• add metadata._inProgress to classifications, and set it to true when a volunteer first updates an annotation.
• set classification.metadata.started_at to the current time when metadata._inProgress changes from false to true.

Staging branch URL: https://pr-{NUMBER}.pfe-preview.zooniverse.org

Fixes #7215.

This is zooniverse/front-end-monorepo#6449 adapted for PFE's redux store. It updates classification.metadata.started_at the first time that you interact with a subject, eg. by answering a question or drawing a mark from a drawing task.

Required Manual Testing

• Does the non-logged in home page render correctly?
• Does the logged in home page render correctly?
• Does the projects page render correctly?
• Can you load project home pages?
• Can you load the classification page?
• Can you submit a classification?
• Does talk load correctly?
• Can you post a talk comment?

Review Checklist

• Does it work in all major browsers: Firefox, Chrome, Edge, Safari?
• Does it work on mobile?
• Can you npm ci and app works as expected?
• If the component is in coffeescript, is it converted to ES6? Is it free of eslint errors? Is the conversion its own commit?
• Are the tests passing locally and on GitHub Actions?

Optional

• Have you replaced any ChangeListener or PromiseRenderer components with code that updates component state?
• If changes are made to the classifier, does the dev classifier still work?
• Have you resized and compressed any image you've added?
• Have you added in flow type annotations?
• Have you followed the Springer guidelines for commit messages?

[coveralls]

coverage: 56.652% (-0.007%) from 56.659%
when pulling 89b1650 on eatyourgreens:classification-started-at
into 131d101 on zooniverse:master.

[eatyourgreens]

The underscore here ensures that the Panoptes JS Client will automatically remove this property when it saves the classification to the API.
https://github.com/zooniverse/panoptes-javascript-client/blob/721b696dccecf86a35614244ab29d19d1a62c3b7/lib/json-api-client/model.js#L17-L38