Subscribers page review

- added screen option to set number of items per page
- improved bulk actions in order to handle large sets

assets/js/src/subscribers/list.jsx

@@ -313,6 +313,7 @@ const SubscriberList = React.createClass({
         </h2>
 
         <Listing
+          limit={ mailpoet_listing_per_page }
           location={ this.props.location }
           params={ this.props.params }
           endpoint="subscribers"