Memoizing root_ancestor in Namespace

The root_ancestor method is heavily used in EE for the group_saml
feature. Having this memoization implemented in CE would eliminate
the need of overriding the root_ancestor method in EE.

app/models/namespace.rb

@@ -250,7 +250,9 @@ class Namespace < ApplicationRecord
   end
 
   def root_ancestor
-    self_and_ancestors.reorder(nil).find_by(parent_id: nil)
+    strong_memoize(:root_ancestor) do
+      self_and_ancestors.reorder(nil).find_by(parent_id: nil)
+    end
   end
 
   def subgroup?