Closed visits 84122476

Add a checkbox to approved visit requests to notify visitors when the visit will be a 'closed visit'.

@jasiek please look at the [first commit](https://github.com/ministryofjustice/prison-visits/commit/17067293fa5c0ce96e73f9ae8726bd4fb56734be) first as I'm not convinced I've implemented the best way to replicate the token.