Add number_of_items property to general inspection object in inspections endpoint

[JuliaBD]

Goal: show the number of items for each inspection in the Diary list (for the user to have an idea how elaborate the inspection is) without having to retrieve the bulky items_by_date.

Other idea: add 'name' property to general inspection, for the user to easily label (and find) inspections. I could make a separate item for this?