Feat 27: Add linter and unit test frameworks

[helen-m-lin]

closes #27

• Configured Jest test runner with React Testing Library
  • use npm run test:coverage or npm run test (watch mode)
  • configured coverage thresholds to current coverage
• Added and configured ESLint for React, with JS Standard Code Style
  • use npm run lint or npm run lint:fix (auto-fixes issues)
  • fixed all current linting errors
  • Updated README.md as appropriate

NOTE: there will be future tickets to increase test coverage to 100%, add pre-commit/ pre-push Git hooks to enforce linter and unit tests, and add GitHub Actions to enforce the same on PRs