[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

app/views/search/_category.html.haml

@@ -100,8 +100,9 @@
             Wiki
             %span.badge.badge-pill
               = limited_count(@search_results.wiki_blobs_count)
-      %li{ class: active_when(@scope == 'users') }
-        = link_to search_filter_path(scope: 'users') do
-          Users
-          %span.badge.badge-pill
-            = limited_count(@search_results.limited_users_count)
+      - if can?(current_user, :read_users_list)
+        %li{ class: active_when(@scope == 'users') }
+          = link_to search_filter_path(scope: 'users') do
+            Users
+            %span.badge.badge-pill
+              = limited_count(@search_results.limited_users_count)