Check storage in local "done" folder on completed transfer

On a completed transfer, check the DONE folder of the managed location.

Assemble information like this:

• per user-directory
• parse age (creation time) of each transfer subdirectory
• sum up size of those sub-dirs older than a given threshold (GraceExpire)
• compile into an email to either the admins or a specific GraceNotification email address

For the follow-up cleanup task see issue #6.