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.

@hwesta, are you okay with pushing base64 changes deeper into the filesystem browser JS code like this?