Ensure paths are treated internally as base64 everywhere

* filesystem_ajax now also uses base64 between the dashboard and itself,
  instead of doing a translation before paths are sent to the SS.
* directory_to_dict path helper also translates parent directories.
* filesystem browser uses base64 paths everywhere outside of HTML
  representation.

refs #6067

This reverts commit 69bc5a0617198fe8d8078c1bab5fc7250dcfb4a3.
filesystem/download: accept base64-encoded paths

refs #6067