Improve scrolling behaviour (#1415)

* Replace column margin with padding

This improves horizontal scrolling behaviour significantly; scrolled flex elements are... a little weird.

* Move clear column button styling to css

app/assets/javascripts/components/features/notifications/components/clear_column_button.jsx

@@ -4,16 +4,6 @@ const messages = defineMessages({
   clear: { id: 'notifications.clear', defaultMessage: 'Clear notifications' }
 });
 
-const iconStyle = {
-  fontSize: '16px',
-  padding: '15px',
-  position: 'absolute',
-  right: '48px',
-  top: '0',
-  cursor: 'pointer',
-  zIndex: '2'
-};
-
 const ClearColumnButton = React.createClass({
 
   propTypes: {
@@ -25,7 +15,7 @@ const ClearColumnButton = React.createClass({
     const { intl } = this.props;
 
     return (
-      <div title={intl.formatMessage(messages.clear)} className='column-icon' tabIndex='0' style={iconStyle} onClick={this.props.onClick}>
+      <div title={intl.formatMessage(messages.clear)} className='column-icon column-icon-clear' tabIndex='0' onClick={this.props.onClick}>
         <i className='fa fa-eraser' />
       </div>
     );