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.
Fixed: this will return list of [id] instead of single unicode id                  

--  
Aelxander Kryklia
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On Friday, August 22, 2014 at 12:47 PM, Tim Babych wrote:

> In common/lib/xmodule/xmodule/split_test_module.py:
> > child_descriptor = self.get_child_descriptor_by_location(child_location) > child = self.system.get_module(child_descriptor) > rendered_child = child.render(STUDENT_VIEW, context) > fragment.add_frag_resources(rendered_child) > - > - contents.append({ > - 'group_id': group_id, > + group_name, updated_group_id = self.get_data_for_vertical(child) > + > + if updated_group_id is None: # inactive group > + group_name = child.display_name > + updated_group_id = [g_id for g_id, loc in self.group_id_to_child.items() if loc == child_location]  
> this will return list of [id] instead of single unicode id
>  
> —
> Reply to this email directly or view it on GitHub (https://github.com/edx/edx-platform/pull/4888/files#r16589881).
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