Clear item selection on delete

When you delete a selection of items
(folders/files), the selected items
are cleared and the Toolbar hides.

fixes CNVS-16856

Test Plan:
* Enable "Better File Browsing" feature flag
* Go to any course's Files page
* Select multiple files/folders
* Click the "Delete" button in the Toolbar
* Confirm the deletion on the popup
* The items should be deleted and the Toolbar
  should disappear

Change-Id: Ic2d37f43399e94dd2a91fbf0c1130b75c86085bb
Reviewed-on: https://gerrit.instructure.com/44494
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Sterling Cobb <sterling@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Dan Minkevitch <dan@instructure.com>