SSDM-13579: Added a check in the pre-flight requests (OPTIONS) branch, which is required for a DELETE request. This is to make it do a real request check instead of merely accepting everything.

SSDM-13579: Made the server accept pre-flight requests (OPTIONS), which is required for a DELETE request.

SSDM-13579: Made the LeftToolbar component change depending on whether at least one file is selected or not.