Report empty directories from a recursive /list.

This allows downloaders to reproduce empty directories within the registry, and
is consistent with the recording of such directories in the manifest.

Also did a minor bugfix to the empty directory discovery code in the walker, to
avoid reporting directories that only contain other empty directories.

Mimic move with copy-and-delete to reset file permissions when consum…

Reindexing should only be performed by administrators.

Because it modifies existing registry contents, which is a serious operation.
This change also ensures that we're consistent with the documentation.

Bugfix to upload locks to protect access to asset permissions.

We do some refactoring to make it more obvious that the asset-level locks are
properly acquired in all code paths. This involves some duplication of the
permission-checking code so we add some extra tests.

Forbid 'source' names that are not directories during upload.

This protects against uploads of the contents of symbolic links.

Tidied up tests to use subtests instead of a loop.

Include the json property names in the rerouteActions class.

Also cleaned up the docs for the rerouting properties.

Added option to forcibly delete versions/assets or reject probational…

Remove all existing (potentially obsolete) ..links files during reind…