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/compose/components/privacy_dropdown.jsx

@@ -83,7 +83,7 @@ const PrivacyDropdown = React.createClass({
         <div className='privacy-dropdown__value'><IconButton icon={valueOption.icon} title={intl.formatMessage(messages.change_privacy)} size={18} active={open} inverted onClick={this.handleToggle} style={iconStyle} /></div>
         <div className='privacy-dropdown__dropdown'>
           {options.map(item =>
-            <div key={item.value} onClick={this.handleClick.bind(this, item.value)} className={`privacy-dropdown__option ${item.value === value ? 'active' : ''}`}>
+            <div role='button' tabIndex='0' key={item.value} onClick={this.handleClick.bind(this, item.value)} className={`privacy-dropdown__option ${item.value === value ? 'active' : ''}`}>
               <div className='privacy-dropdown__option__icon'><i className={`fa fa-fw fa-${item.icon}`} /></div>
               <div className='privacy-dropdown__option__content'>
                 <strong>{item.shortText}</strong>