Introduce a warning message in Children Display macro configuration when the Parent Page have a lot of Child Page.

Due to the bug reported in CONFSRVDEV-7847 and CONFSERVER-43191, it will be useful for Confluence user to avoid adding the Children Display macro for pages that contains a lot of child pages (~3000) if we can introduce a warning message when the user specify the Parent page that contains a lot of child page inside the Children Display macro configuration.

Example message:

The Parent Page contains xxxx of child pages. Adding this macro will delay the macro load time.