Resolve "Inverse index on namespaces parent_id, id"

What does this MR do?

Add an index: create index idx_namespaces_and_child_groups on namespaces (id) where parent_id IS NOT NULL

As recommend by !57318 (comment 566770357) in order to walk over the namespaces table.

== 20210506064413 CreateNamespacesIdParentIdInversePartialIndex: migrating ====
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:where=>"parent_id IS NOT NULL", :name=>"index_namespaces_id_parent_id_is_not_null", :algorithm=>:concurrently})
   -> 0.0118s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:namespaces, :id, {:where=>"parent_id IS NOT NULL", :name=>"index_namespaces_id_parent_id_is_not_null", :algorithm=>:concurrently})
   -> 0.0038s
-- execute("RESET ALL")
   -> 0.0005s
== 20210506064413 CreateNamespacesIdParentIdInversePartialIndex: migrated (0.0177s)

== 20210506064413 CreateNamespacesIdParentIdInversePartialIndex: reverting ====
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:name=>"index_namespaces_id_parent_id_is_not_null", :algorithm=>:concurrently})
   -> 0.0077s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:namespaces, {:name=>"index_namespaces_id_parent_id_is_not_null", :algorithm=>:concurrently, :column=>:id})
   -> 0.0093s
-- execute("RESET ALL")
   -> 0.0006s
== 20210506064413 CreateNamespacesIdParentIdInversePartialIndex: reverted (0.0191s)

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Related to #330086 (closed)