Use parent group as sidebar context for new subgroups and projects

What does this MR do and why?

This fixes a bug in the new navigation alpha. Show group sidebar while creating a new subgrou... (!109649 - merged) wasn't written with !111186 (merged) in mind. So with both MRs merged and the new nav toggled on, creating a new subgroup or -project raises an error while trying to render the group sidebar with the wrong, unsaved group (with id being nil, etc.)

This MR applies the same logic to the new nav as !109649 (merged) did to the current nav, where we use the @parent_group if it set.

Screenshots or screen recordings

How to set up and validate locally

• Check http://gdk.test:3000/groups/new?parent_id=31#create-group-pane
• Check http://gdk.test:3000/projects/new?parent_id=31

Both should render the group sidebar of the parent group.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.