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/components/load_more.jsx

@@ -1,7 +1,7 @@
 import { FormattedMessage } from 'react-intl';
 
 const LoadMore = ({ onClick }) => (
-  <a href='#' className='load-more' onClick={onClick}>
+  <a href="#" className='load-more' role='button' onClick={onClick}>
     <FormattedMessage id='status.load_more' defaultMessage='Load more' />
   </a>
 );