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/column_header.jsx

@@ -25,7 +25,7 @@ const ColumnHeader = React.createClass({
     }
 
     return (
-      <div aria-label={type} className={`column-header ${active ? 'active' : ''}`} onClick={this.handleClick}>
+      <div role='button' tabIndex='0' aria-label={type} className={`column-header ${active ? 'active' : ''}`} onClick={this.handleClick}>
         {icon}
         {type}
       </div>