Fav button into ES6

[srallen]

Converts the favorites button component into ES6 and adds unit tests. Fixed a bug in show-list.cjsx if the favorites collection does not have any subject links.

https://fav-button-es6.pfe-preview.zooniverse.org/

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?
• Did you deploy a staging branch?
• Did you get any type checking errors from Babel Typecheck

Optional

• If it's a new component, is it in ES6? Is it clear of warnings from ESLint?
• 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 added in flow type annotations?

[mention-bot]

@srallen, thanks for your PR! By analyzing the history of the files in this pull request, we identified @eatyourgreens, @amyrebecca and @camallen to be potential reviewers.

[eatyourgreens]

Add aria-label={(this.state.favorited) ? 'Unfavorite' : 'Favorite'} here since the button doesn't have a text label.

[eatyourgreens]

Looks good to me. One suggestion about adding a text label to the button.