Scandipal: added more information about the division/grouping of the itemparts and their display_label in the admin. Added Image.custom_label which allows the editor to override the auto-generated Image.display_label. Added ItemPart.group_locus to record the location of a part within another part. Reviewed the logic behind ItemPart.display_label: it takes the CI.shelfmark + locus by default and HI.name + locus OR GroupItemPart.HI.name + group_locus otherwise.