Include source namespace when indexing users into Elasticsearch

What does this MR do and why?

Includes member => source => namespace when preloading user data for indexing into Elasticsearch. This gets rid of an additional namespace query that's needed when a user has a project namespace.

Before

Source is being preloaded but we see additional queries for namespaces:

After

Now the namespace of source is also preloaded and the only additional queries are to recursively/linearly determine namespace ancestors:

How to set up and validate locally

service = Elastic::ProcessBookkeepingService.new
Elastic::ProcessBookkeepingService.track!(*User.all)
ActiveRecord::Base.logger = Logger.new $stdout  

items = Elastic::ProcessBookkeepingService.queued_items
refs = service.send(:deserialize_all, items.values.flatten(1))

refs.preload_database_records.each do |ref|
  puts ">>>>>>>>>>> #{ref.klass.to_s}"
  service.send(:submit_document, ref)
end;nil

Take note of the queries before the first >>>>>>>>>>> which is the preloading. Any queries after that are additional queries.

MR acceptance checklist

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

Related to #390267 (closed)