UX: Consolidate "Admins" and "Moderators" into "Staff" on admin users

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lib/admin_user_index_query.rb

@@ -49,6 +49,7 @@ class AdminUserIndexQuery
 
   def filter_by_query_classification
     case params[:query]
+      when 'staff'      then @query.where("admin or moderator")
       when 'admins'     then @query.where(admin: true)
       when 'moderators' then @query.where(moderator: true)
       when 'blocked'    then @query.blocked