Ref T6013. Currently, when we create a thumbnail, it gets its own (default) file visibility policy.

In particular, this causes the issue in T6013: thumbnails get "all users" visibility, which does not include logged-out users.

Instead, a thumbnail should just have the same visibility as the original file does. Enforce this:

• When loading thumbnails, reject thumbnails with invisible originals.
• When filtering thumbnails, permit thumbnails with visible originals.

As a logged-out user, thumbnails are now visible when the original files are attached to visible objects.