Improved notifications cleaning UI with set operations (#109)

* added notification cleaning drawer

* bugfix

* fully implemented set operations for notif cleaning

* i18n for notif cleaning drawer & improved logic slightly. Also added a confirm dialog

* - notif dismiss "overlay" now shoves the notif aside to avoid overlap
- added focus ring to header buttons
- removed notif overlay entirely from DOM if mode is disabled

* removed comment

* CSS tuning - inconsistent division lines fix

app/javascript/mastodon/components/column.js

@@ -7,6 +7,7 @@ export default class Column extends React.PureComponent {
 
   static propTypes = {
     children: PropTypes.node,
+    extraClasses: PropTypes.string,
   };
 
   scrollTop () {
@@ -40,10 +41,10 @@ export default class Column extends React.PureComponent {
   }
 
   render () {
-    const { children } = this.props;
+    const { children, extraClasses } = this.props;
 
     return (
-      <div role='region' className='column' ref={this.setRef}>
+      <div role='region' className={`column ${extraClasses || ''}`} ref={this.setRef}>
         {children}
       </div>
     );