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.
Thank you, fixed: "I do think the initial creation of the group configuration and the initial assignment of a content experiment to the group configuration could be simplified"

I'm not sure I understood you how can I simplify publishing part.