Add DOB and DOD support

Items that lack DOB and DOD are added to the "people_no_date" table without distinction, which means that an item without a DOD will be returned as having neither a DOB or DOD. To fix this, we'll need to create two new tables – I've used `people_no_dob` and `people_no_dod` – with the same schema as `people_no_date` and then populate them by running the `tools/update_people_no_date` script.