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_collapsable.jsx

@@ -46,7 +46,9 @@ const ColumnCollapsable = React.createClass({
 
     return (
       <div style={{ position: 'relative' }}>
-        <div title={`${title}`} style={{...iconStyle }} className={`column-icon ${collapsedClassName}`} onClick={this.handleToggleCollapsed}><i className={`fa fa-${icon}`} /></div>
+        <div role='button' tabIndex='0' title={`${title}`} style={{...iconStyle }} className={`column-icon ${collapsedClassName}`} onClick={this.handleToggleCollapsed}>
+          <i className={`fa fa-${icon}`} />
+        </div>
 
         <Motion defaultStyle={{ opacity: 0, height: 0 }} style={{ opacity: spring(collapsed ? 0 : 100), height: spring(collapsed ? 0 : fullHeight, collapsed ? undefined : { stiffness: 150, damping: 9 }) }}>
           {({ opacity, height }) =>