Bottom padding added to drupal-off-canvas dialog

20px bottom padding added to the off-canvas dialog.

Change applied to both /core/misc/dialog/off-canvas.css and /core/themes/stable/css/core/dialog/off-canvas.css

Before:

 .ui-dialog-off-canvas #drupal-off-canvas {
   padding: 0 20px;
 }

After:

 .ui-dialog-off-canvas #drupal-off-canvas {
   padding: 0 20px 20px;
 }