Allow disabling Facebook warning

As a maintainer, I agree that this suggestion makes a lot of sense. Perhaps the best thing to do is to move that warning to the URL Embed configuration section, instead of the generic site warnings.

That makes sense to me. If a site builder sees that Facebook embeds do not work, they might visit the url_embed config page to find out why and see the warning there but the warning does not appear on the main status page.

I think it would make sense that if facebook_app_id or facebook_app_secret are provided that the url_embed_debug_facebook_access_token() check still runs. In other words, no warning if they are not configured but if they are there, they must be valid. Other than that, reading the code, it looks good.