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.jsx

@@ -15,13 +15,13 @@ const ColumnBackButton = React.createClass({
   mixins: [PureRenderMixin],
 
   handleClick () {
-    if (window.history && window.history.length == 1) this.context.router.push("/");
+    if (window.history && window.history.length === 1) this.context.router.push("/");
     else this.context.router.goBack();
   },
 
   render () {
     return (
-      <div onClick={this.handleClick} className='column-back-button'>
+      <div role='button' tabIndex='0' 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>