Upgrade Panoptes client to 2.8.1

[eatyourgreens]

Staging branch URL: https://upgrade-api-client.pfe-preview.zooniverse.org

Fixes #4179.

Fixes bearer token expiration problems with the Talk client.

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?

[simoneduca]

Testing this seems to involve waiting until the token actually expires. Any alternative thoughts?

[eatyourgreens]

The steps to replicate on #4179 say "wait a long time…"

[simoneduca]

I was able to reproduce the bug as well as test the fix. LGTM.