Add jsx-a11y linter

[rinti]

This will add a11y linter with their recommended settings. I suggest we try this out for a while and adjust the rules as we go if we find some things too strict.

Example component:

    <div className="Hero">
        <h1 className="Hero__Title">{title}</h1>
        <img src="http://placehold.it/400x400" />
        <a href="#">Link with invalid href</a>
    </div>

And output:

  8:9  error  img elements must have an alt prop, either with meaningful text, or an empty string for decorative images                                                                                                                                                                                                                                                         jsx-a11y/alt-text
  9:9  error  The href attribute requires a valid value to be accessible. Provide a valid, navigable address as the href value. If you cannot provide a valid href, but still need the element to resemble a link, use a button and change it with appropriate styles. Learn more: https://github.com/evcohen/eslint-plugin-jsx-a11y/blob/master/docs/rules/anchor-is-valid.md  jsx-a11y/anchor-is-valid