"Add LastVisit" and "Edit Visit" require an "End Date"

This prevents you from entering retrospective data, where a visit must be active (not have an end date) to be able to certain actions; fill out custom forms, do admissions/discharges, etc.

I don't think the end-date was required when adding a retrospective visit or editing visits on previous version of the Reference Application.

Acceptance Criteria

1. You should be allowed to add a past visit with a start date but no end date
2. But you should not be allowed to add a past visit with a start date but no end date if this would overlap with another existing visit
3. You should be allowed to edit a closed visit and remove its end date
4. But you should not be allowed to edit a closed visit and remove its end date if this would overlap with another existing visit