Add Namespace#feature_available no-op

This gets overridden in `EE::Namespace` and allows us to do things like
always treat Epics as "disabled" in Core using the same checks we'd use
elsewhere.

app/models/namespace.rb

@@ -263,6 +263,11 @@ class Namespace < ApplicationRecord
     false
   end
+  # Overridden in EE::Namespace
+  def feature_available?(_feature)
+    false
+  end
+
   def full_path_before_last_save
     if parent_id_before_last_save.nil?
       path_before_last_save