chroot users by default

[jbygdell]

Currently we just substitute the bucket and the user is forced to put the username first in the upload path.
To simplify the user experience we should add the username from the token when we add in the bucket in the path.

One suggestion is to have everyone upload to the bucket s3://INBOX and then in the s3proxy replace and/or add the userid from the token so the files end up in the right place (s3://INBOX/<userid>/).