fix: undefined project.slug in Talk search results

[eatyourgreens]

Fix an error where project.slug is undefined in Talk search results, leading to raw markdown strings being passed to the HTML parser in the Markdown component.

Staging branch URL: https://pr-7068.pfe-preview.zooniverse.org/talk/search?page=11&query=add&env=production

• Fixes #7067.

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: 56.98%. remained the same when pulling 0e398cb25800b0e95f8770a38aee68daaff86cb4 on eatyourgreens:escape-talk-comments into dd307b3fc1b33dbbebd50a3df8ffe967fb795418 on zooniverse:master.

[eatyourgreens]

A side effect of this fix is that Talk search results will now be properly formatted again. They've been displayed as raw markdown strings.