Add a hotkey for 'delete marking' - the 'x' key

[tingard]

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

Fixes # .

Describe your changes.

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?

[vercel[bot]]

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[tingard]

Example project URL: /projects/1715labs/satellite-grave-marking-2021-10-18/classify?workflow=20555&subject=71145764&no_tutorial=true&env=production