Simplify VisitorsManipulator a little
Simplify VisitorsManipulator a little

The logic was so tortured that it was hard to work out what was going
on.

BEWARE: It appears to be essential that 'valid?' is called on each
visitor, in order that the side-effect of generating error messages
happens before the view is rendered.

ministryofjustice/prison-visits,ministryofjustice/prison-visits,ministryofjustice/prison-visits