filebackend-user-access

based on irc conversations with aaron schulz and chris steipp:

* use basePath instead of containerPaths
* moved $wgGWTFBMetadataContainer into Config::$filebackend_metadata_container
* the filebackend path in the forms are now relative instead of complete filebackend paths.
* added user name to filebackend path in order to add a layer of user access control and organize user uploaded metadata files

Change-Id: I278d9f3b1332f7a2e1ee6a2d759569ba322db390