Resolve "Remove unpinned items from sidebar for existing users"

What does this MR do and why?

Adds the feature flag hide_unpinned_sidebar_items which will be used in Personalized navigation – Phase 2 (gitlab-org#22184) and especially Remove unpinned items from sidebar for existing... (#601398 - closed)

References

Remove unpinned items from sidebar for existing... (#601398 - closed)

How to set up and validate locally

n/a

MR acceptance checklist

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Related to #601398 (closed)