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: 56.656% (-0.03%) from 56.684%
when pulling 40ee27a on eatyourgreens:classification-started-at
into 8ad3b19 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

[goplayoutside3]

The full comment about why this PR is being closed can be found here: zooniverse/front-end-monorepo#6447 (comment)

Some of the info copied here for clarity:
The frontend components that display hours a volunteer spent classifying use session_time from ERAS. If the classification is stored in Panoptes, session_time is calculated by the classification’s finished_at - started_at metadata. When Ouroboros classifications were ported over to Panoptes, steps were taken by the backend team to account for the lack of finished_at in the metadata.

session_time is where a time-limit cap is applied if the two properties were recorded several hours apart or even days apart. The time-limit cap was decided specifically with the new user stats website features in mind. We decided it was better to overestimate from a place of trust than underestimate time spent classifying in the cases where finished_at - started_at is large.

Decision:
started_at will continue to be recorded on subject load. Changing started_at to the time of first annotation misses the volunteer's time spent analyzing the subject before making an annotation, referencing the tutorial, and watching the flipbook or video subjects. For some projects annotation begins with interaction with the subject through drawing while for others it begins only once a simple choice task is completed. started_at recorded on subject load is in line with “better to overestimate from a place of trust than underestimate”.