UX improvement in gds-sso for remote logout

http://git.io/llA6Qw
govuk_content_models needed to be updated
to get this version of gds-sso through
ApiUsers are Users, so this wasn't working

since api users started being a User using
bearer tokens this code was not of any use
anyways, so removing it.
Merge pull request #117 from alphagov/use-latest-gds-sso

UX improvement in gds-sso for remote logout