Fix count of all group members in autocomplete

What does this MR do and why?

The count shown for the "All Group Members" entry in the user autocomplete should only include direct group members since those are the only ones that get pinged when @all is used at the group-level.

How to set up and validate locally

In an Epic description / comment field, type @all. The count should only include direct group members.

MR acceptance checklist

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