Lint for Accessibility

We want to change our [eslint configuration file](https://github.com/codesupport/website-frontend/blob/master/.eslintrc.js) to also check for accessibility issues using the [jsx-a11y](https://www.npmjs.com/package/eslint-plugin-jsx-a11y) eslint plugin. As well as changing the config, we should fix any issues it notifies us of.