Add eslint-plugin-jsx-a11y (#1651)

* Add eslint-plugin-jsx-a11y.

* Fix npm script.

* Adjust npm scripts so test also runs lint.

* Fix existing lint errors.

* Don't break on a11y issues.

* Add role and tabIndex.

* Add vim and Mac files to .gitignore and .dockerignore.

* Handle htmlFor (partially), a that's actually a button.

* Fix missing tabIndex.

* Add cursor:pointer to load-more

* Revert change to load_more.

* Fixes based on review.

* Update yarn.lock.

* Don't try to install fsevents on Linux (hides warning noise).

app/assets/javascripts/components/features/ui/components/modal_root.jsx

@@ -66,7 +66,7 @@ const ModalRoot = React.createClass({
 
               return (
                 <div key={key}>
-                  <div className='modal-root__overlay' style={{ opacity: style.opacity, transform: `translateZ(0px)` }} onClick={onClose} />
+                  <div role='presentation' className='modal-root__overlay' style={{ opacity: style.opacity, transform: `translateZ(0px)` }} onClick={onClose} />
                   <div className='modal-root__container' style={{ opacity: style.opacity, transform: `translateZ(0px) scale(${style.scale})` }}>
                     <SpecificComponent {...props} onClose={onClose} />
                   </div>