Need to make sure workspace validates path

[jessefreeman]

The WorkspaceService routs all file system calls to the FileSystem service. The workspace should validate that the system path is safe to write to. Need to modify ConvertToSystemPath to only return a value if the path is in a safe directory when writing to the system. If not, don't write file. Go through and make sure this is checked on every API that writes data to the file system.