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.

Here's an example of the cookies now dropped when running Identity and NGW Frontend on my local machine:

![sc_gu_la cookie](https://cloud.githubusercontent.com/assets/52038/4457056/3369256e-4892-11e4-9396-cc3737d43319.png)

https://jira.gutools.co.uk/browse/MEM-403



cc @dbgdngit @adamnfish @mchv