Update cookies
Update cookies

As part of this Trello card (https://trello.com/c/A2gUGB0u/1369-cookies-tidyup)
we are changing the name of the session cookie and setting it to expire
after 20mins.  The T&Cs already had 20mins written in to them.

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