Render only existing children in staff view of split_test.

This fixes BLD-1224.

Root cause is that group_id_to_child contains deleted groups.
Other solution is to clean group_id_to_child of split_test when group was deleted from group configuration page. But this will affect publishing. So this solution was implemented but skipped.

@tymofij @cahrens please review.

Also this provides fix for https://openedx.atlassian.net/browse/BLD-1249.
Nit: it would be cleaner to just always return split_test if associate_experiment is True (instead of introducing a second variable that has to be in sync with associate_experiment). It could be ignored if not used (though this involves touching all the places that create_group_configuration_experiment is called with associate_experiment=True to updated the expected return value).