Improve down_sync_asset hook

[aatanasov-cloudinary]

Approach

• Ensured only logged-in users with the upload_files capability can import assets from the DAM.

QA notes

Cases:

• Go to the Media Library admin page and upload an asset.
• Go to a page and upload an asset.
• Import an asset from the DAM in the Media Library modal (the Cloudinary tab in the modal):

• Go to the Cloudinary DAM page and import an asset from the DAM.

In all of those cases, the assets should be synced.