Add unique users for achievements

What does this MR do and why?

Adds a uniqueUsers field to the achievement type so we can display a list of user avatars on the "manage group achievements" page.

Belongs to #568853 Backend part of unique users for achievements. Frontend part: !208549

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the achievements feature flag
2. Navigate to a group achievements page: http://gdk.test:3000/groups/flightjs/-/achievements
3. Create an achievement or use an existing one
4. Award the achievement to the same user multiple times
5. Run a GraphQL query to ensure a unique list of recipients is returned:

query getGroupAchievements{
  group(fullPath: "flight") {
    id
    achievements {
      nodes {
        name
        uniqueUsers {
          nodes {
            username
          }
        }
      }
    }
  }
}

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