Fix index namespaces for indexing subgroup associations

What does this MR do and why?

Related to #477279 (closed)

This MR fixes a bug where subgroup epics are not being indexed with the gitlab:elastic:index rake task

MR acceptance checklist

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Screenshots or screen recordings

N/A

How to set up and validate locally

to speed up the index rake task for dev environments, apply the patch below (I'm adding this in another MR)

Click to expand patch

diff --git a/ee/app/workers/search/elastic/trigger_indexing_worker.rb b/ee/app/workers/search/elastic/trigger_indexing_worker.rb
index 6122613e16fd..525cdf1520f6 100644
--- a/ee/app/workers/search/elastic/trigger_indexing_worker.rb
+++ b/ee/app/workers/search/elastic/trigger_indexing_worker.rb
@@ -9,6 +9,7 @@ class TriggerIndexingWorker
 
       INITIAL_TASK = :initiate
       TASKS = %i[namespaces projects snippets users].freeze
+      DEFAULT_DELAY = 2.minutes
 
       data_consistency :delayed
 
@@ -55,15 +56,14 @@ def initiate
           ).execute
 
           logger.info('Setting `elasticsearch_indexing` has been enabled.')
-          self.class.perform_in(2.minutes, INITIAL_TASK, options)
+          reenqueue_initial_task
 
           return false
         end
 
         unless ::Gitlab::CurrentSettings.elasticsearch_pause_indexing?
           task_executor_service.execute(:pause_indexing)
-
-          self.class.perform_in(2.minutes, INITIAL_TASK, options)
+          reenqueue_initial_task
 
           return false
         end
@@ -87,6 +87,14 @@ def task_executor_service
       def logger
         @logger ||= ::Gitlab::Elasticsearch::Logger.build
       end
+
+      def reenqueue_initial_task
+        if Rails.env.development?
+          self.class.perform_async(INITIAL_TASK, options)
+        else
+          self.class.perform_in(DEFAULT_DELAY, INITIAL_TASK, options)
+        end
+      end
     end
   end
 end
diff --git a/ee/lib/gitlab/elastic/group_search_results.rb b/ee/lib/gitlab/elastic/group_search_results.rb
index e83c026ffb82..2d075f072ff6 100644
--- a/ee/lib/gitlab/elastic/group_search_results.rb
+++ b/ee/lib/gitlab/elastic/group_search_results.rb
@@ -31,7 +31,7 @@ def scope_options(scope)
         case scope
         when :users
           super.except(:group_ids) # User uses group_id for namespace_query
-        when :wiki_blobs, :work_items
+        when :wiki_blobs, :work_items, :epics
           super.merge(root_ancestor_ids: [group.root_ancestor.id])
         else
           super

1. enable elasticsearch for gdk
2. add a group, and add a subgroup (or use an existing subgroup)
3. add an epic to the subgroup
4. run the rake task gitlab:elastic:index
5. make sure that the epic from the subgroup is indexed and findable

Note: I tested this on master and the subgroup epic was not findable (even when limiting is not enabled).