Resolve "Index namespaces parent_id, id"

What does this MR do?

Add an index: create index idx_namespaces_and_top_level_groups on namespaces (id) where parent_id IS NULL

As recommend by !57318 (comment 542943415) in order to walk over namespaces table.

== 20210406063442 CreateNamespacesIdParentIdPartialIndex: migrating ===========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:where=>"parent_id IS NULL", :name=>"index_namespaces_id_parent_id_is_null", :algorithm=>:concurrently})
   -> 0.0096s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- add_index(:namespaces, :id, {:where=>"parent_id IS NULL", :name=>"index_namespaces_id_parent_id_is_null", :algorithm=>:concurrently})
   -> 0.0049s
-- execute("RESET ALL")
   -> 0.0005s
== 20210406063442 CreateNamespacesIdParentIdPartialIndex: migrated (0.0163s) ==

== 20210406063442 CreateNamespacesIdParentIdPartialIndex: reverting ===========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:name=>"index_namespaces_id_parent_id_is_null", :algorithm=>:concurrently})
   -> 0.0075s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:namespaces, {:name=>"index_namespaces_id_parent_id_is_null", :algorithm=>:concurrently, :column=>:id})
   -> 0.0091s
-- execute("RESET ALL")
   -> 0.0005s
== 20210406063442 CreateNamespacesIdParentIdPartialIndex: reverted (0.0187s) ==

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Related to #326637 (closed)