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/column_back_button_slim.jsx

@@ -31,7 +31,7 @@ const ColumnBackButtonSlim = React.createClass({
   render () {
     return (
       <div style={{ position: 'relative' }}>
-        <div style={outerStyle} onClick={this.handleClick} className='column-back-button'>
+        <div role='button' tabIndex='0' style={outerStyle} onClick={this.handleClick} className='column-back-button'>
           <i className='fa fa-fw fa-chevron-left' style={iconStyle} />
           <FormattedMessage id='column_back_button.label' defaultMessage='Back' />
         </div>