'Complete registration of newly-registered user
Allocates device_id if one was not given; also creates access_token.
Args:
(str) user_id: full canonical @user:id
(object) params: registration parameters, from which we pull
device_id and initial_device_name
Returns:
defer.Deferred: (object) dictionary for response from /register'

@defer.inlineCallbacks
def _create_registration_details(self, user_id, params):

    device_id = (yield self._register_device(user_id, params))
    access_token = (yield self.auth_handler.get_access_token_for_user_id(user_id, device_id=device_id, initial_display_name=params.get('initial_device_display_name')))
    defer.returnValue({'user_id': user_id, 'access_token': access_token, 'home_server': self.hs.hostname, 'device_id': device_id})
