Fixed ext storage free space to not be restricted by quota
Fixed storage stats to be based on current directory

Previously, the storage statistics were always for the root dir.
This means that the upload button would always show the limit for the
root dir, even when uploading to a shared dir or external storage.

This fix adds a "dir" argument to getstoragestats.php.

Partial backport of 31181e4348b9af2625cf4d6ad38cf4cd81db3c1f