Add Arkivum

Add Arkivum as a Space.

Also, update ```post_move_from_storage_service``` to do the same path munging that ```move_from_storage_service``` does, and move deleting the staging copy to after ```post_move_from_storage_service``` runs.

This requires #38 
Make sure we log that this comes from the Arkivum class? Otherwise it would be easy to miss context.