track licenses of dependencies
pmount USB device w mode 755 instead of 700
calculate coll/entity root and path_rel

root = path where collections/entities stored (ex: /media/USBHDD/ddr)
path_rel = path to collection/entity within the root
(ex: ddr-densho-1, ddr-densho-1/files/ddr-densho-1-1)
entity.path_rel is useful for calculating file.urls for
sorl-thumbnail