Only clear interventions on Intervention unmount

[eatyourgreens]

Staging branch URL: https://pr-5349.pfe-preview.zooniverse.org

Expect that the Intervention component will not call props.unMount on props change. Update useEffect to pass that test.

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 rm -rf node_modules/ && npm install 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 Travis?

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 remained the same at 48.991% when pulling 4da31db on intervention-unmount into cf78fab on master.

[camallen]

LGTM - tested locally and I don't get crashes anymore

[eatyourgreens]

I'm ambivalent about the comment duplicating the Hooks documentation, which is still new-ish and might change. Can't hurt though.