Set matching persistence-time on cookies generated during re-auth

Now check whether the existing GU_U cookie is persistent, and match that
when generating the new cookies after reauthentication (we don't have a
'Remember Me' checkbox on the re-auth page).
Merge pull request #6468 from guardian/match-persistence-of-existing-cookie-on-reauth

Set matching persistence-time on cookies generated during re-auth