Add the signin permission to the SSO Push User

Currently the SSO Push User only has the `user_update_permission` permission, however due to GDS::SSO not skipping the requirement for the `signin` permission explicitly for the push requests, there is inconsistent behaviour with each app depending on whether the `signin` permission is required at the level of the `ApplicationController`.

The easiest way to fix this right now is to give the SSO User the `signin` permission too. We may want to make a change to the SSO gem in the future to not require this.