Respect the `sessionCookie` flag for the SC_GU_LA cookie [#MEM-403]

This is part of the work to enforce that users are recently signed-in
when they view their personal profile:

https://profile.theguardian.com/account/edit

The Identity API will now be proving Cookie info for the new SC_GU_LA
cookie, which stores when the user last authenticated:

https://github.com/guardian/identity/pull/176

...this change to NGW Frontend is required to get it to respect the new
`sessionCookie` flag, so that the SC_GU_LA cookie is only session-scoped
and dies when the user closes the browser.

https://jira.gutools.co.uk/browse/MEM-403

Added `sessionCookie` default of None to make the tests pass... had to
revert this PR last time:

https://github.com/guardian/frontend/pull/6193
Merge pull request #6221 from guardian/support-session-scoped-identity-cookies

Respect the `sessionCookie` flag for the SC_GU_LA cookie [#MEM-403]