replace Facebook SDK for PHP with app access tokens

Server-to-server requests using app access tokens are preferred over an active Facebook client session using the Facebook SDK for PHP.

Convert additional plugin calls to app access token requests:

- Facebook user permissions
- Facebook friend mentions
- Facebook page mentions

Publish to page setting validation continues to use the Facebook SDK for PHP due to the user access token requirement of the accounts API.

The publish to Facebook timeline override on the WordPress profile page is only shown if the profile's Facebook account has granted the site's Facebook application `publish_actions` permission.