Fix auth-call to Identity API: ask for appropriate cookie persistence

Frontend Identity should pass the 'persistent' parameter to the Identity
API when authenticating:

* [NGW](https://github.com/guardian/frontend/blob/d2a26a768d/identity/app/idapiclient/IdApi.scala#L37-L42) - note that only `format` got sent
* [Identity API](https://github.com/guardian/identity/blob/d85a787fcb/identity-api/src/main/scala/com/gu/identity/api/servlet/AuthServlet.scala#L49) - note the API takes both `format` _and_ `persistent` parameters

All `GU_U` cookies dropped by Frontend Identity up till now have had their
`persistent` flag (the 6th value in the cookie) set to **`0`** rather than
`1`, even if you ticked 'Remember Me' when you signed in. The cookies
[still got the correct lifespan](https://github.com/guardian/frontend/blob/420f585f/identity/app/services/PlaySigninService.scala#L18)
but they didn't contain the right _value_ for the 'persistent' field.

https://github.com/guardian/frontend/pull/6468#issuecomment-59018464
Merge pull request #6480 from guardian/pass-remember-me-value-to-identity-api

Fix auth-call to Identity API: ask for appropriate cookie persistence [#MEM-403]