add Facebook app debug output

Add a new Facebook app section to the debugger section of plugin settings for plugins with a verified app_id and app_secret combo.

Request Facebook application settings values related to the Facebook login permissions dialog. Display those values on the page or highlight the lack of a stored value and suggest a possible value from a similar WordPress setting.

Inform the current WordPress user if he or she has the required permissions to edit these values on the Facebook Developers site or suggest other site users who could make the edit.