[EE] Add users search results to global search

What does this MR do?

EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21197

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

lib/gitlab/group_search_results.rb

@@ -13,11 +13,17 @@ def users
       # 1: get all groups the current user has access to
       groups = GroupsFinder.new(current_user).execute.joins(:users)
 
-      # 2: get all users the current user has access to (-> `SearchResults#users`)
+      # 2: Get the group's whole hierarchy
+      group_users = @group.direct_and_indirect_users
+
+      # 3: get all users the current user has access to (->
+      # `SearchResults#users`), which also applies the query.
       users = super
 
-      # 3: filter for users that belong to the previously selected groups
-      users.where(id: groups.select('members.user_id'))
+      # 4: filter for users that belong to the previously selected groups
+      users
+        .where(id: group_users.select('id'))
+        .where(id: groups.select('members.user_id'))
     end
     # rubocop:enable CodeReuse/ActiveRecord
   end