PERF: Cache categories in Site model take 3.

Previous attempt resulted in custom fields going missing in the
serialized output.

This reverts commit 83a6ad32ff.

app/models/concerns/has_custom_fields.rb

@@ -142,6 +142,11 @@ module HasCustomFields
     super
   end
 
+  def on_custom_fields_change
+    # Callback when custom fields have changed
+    # Override in model
+  end
+
   def custom_fields_preloaded?
     !!@preloaded_custom_fields
   end
@@ -197,8 +202,11 @@ module HasCustomFields
       if row_count == 0
         _custom_fields.create!(name: k, value: v)
       end
+
       custom_fields[k.to_s] = v # We normalize custom_fields as strings
     end
+
+    on_custom_fields_change
   end
 
   def save_custom_fields(force = false)
@@ -253,6 +261,7 @@ module HasCustomFields
         end
       end
 
+      on_custom_fields_change
       refresh_custom_fields_from_db
     end
   end