Enable advanced user search regardless of namespace

What does this MR do and why?

Enables advanced search for the users scope even if namespace limiting is enabled.

The purpose of the change is to enable advanced search for all users who are able to use Elasticsearch to reduce the number of basic searches.

How

Adds a method to elasticsearchable.rb which is set to true when the scope is users. This method is then used in Search::GroupService and Search::ProjectService to determine if elasticsearchable_scope should be set or not (group for GroupService and project for ProjectService).

ApplicationSetting determines if the search should be done using Elasticsearch or not based on elasticsearchable_scope if namespace limiting is enabled.

This results in:

• if the scope is users, elasticsearchable_scope is always nil which means even if namespace limiting is enabled, advanced search will be performed.
• if the scope is not users, nothing changes.

Impact

We expect less basic searches when a group or project is specified which accounts for most of the basic searches that take longer than 5 seconds:

How to set up and validate locally

1. Ensure Elasticsearch is set up locally.
2. Enable namespace limiting (go to /admin/application_settings/advanced_search and check the "Limit the number of namespaces and projects that can be indexed" box.
3. Set feature flags: ::Feature.enable(:advanced_user_search) and ::Feature.enable(:advanced_global_search_for_limited_indexing).
4. Perform a user search: /search?scope=users&search=reported_user and verify advanced search is used.
5. Select a group and verify advanced search is used.
6. Select a project and verify advanced search is used.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #390337 (closed)