[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

@@ -10,7 +10,14 @@ def initialize(current_user, limit_projects, group, query, default_project_filte
 
     # rubocop:disable CodeReuse/ActiveRecord
     def users
-      super.where(id: @group.users_with_descendants)
+      # 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`)
+      users = super
+
+      # 3: filter for users that belong to the previously selected groups
+      users.where(id: groups.select('members.user_id'))
     end
     # rubocop:enable CodeReuse/ActiveRecord
   end