Public volume export

[mzur]

This module already offers a public label tree export where all sensitive user information (i.e. password hashes) are removed. We could also offer a public volume export so users can archive their volumes externally.

[mzur]

This is a duplicate of biigle/sync#21, which was closed because if iFDO support. I'd like to reopen this issue because a public volume export (with a compatible import) may be desirable even if no iFDO is available. This is very similar to the public label tree export/import.

[mzur]

With #709 it will be possible to offer an import of exported volumes, too! But this may no longer be necessary with the full iFDOv2 import and export (unless there is some information that is not included in there?).