Adds the latest version of ESLint and Prettier, and corrects subsequent errors and warnings.

Additions

• ESLint
• Prettier

Changes

• Modifies existing code to meet ESLint rules, esp. variable name length and unnamed default exports
• Disables rules where implementation would be counterproductive or unnecessary
• Fleshes out JSDOC statements

Testing

• yarn test should pass
• yarn lint should pass
• Browse through app and check that it works and looks the same. Please pay particular attention to icons and buttons/anchors as some refactoring was necessary that might have affected styling.

Checklist

• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the front end playbook
• Passes all existing automated tests
• New functions include new tests
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged
• Visually tested in supported browsers and devices
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)