Added ability for zoekt ui to search globally

What does this MR do and why?

Added ability for zoekt ui to search globally

References

• #534835 (comment 2439764088)+

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make sure advanced search is installed and enabled
2. Make sure you enable zoekt.
3. enable feature flag

  echo 'Feature.enable(:zoekt_cross_namespace_search)' | rails c

1. Visit search page http://gdk.test:3000/search?scope=blobs&search=test
2. Search code globally (no group or project filter is applied)
3. With the FF enabled you should be able to search multi-result.

MR acceptance checklist

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