Use app access token for Facebook requests without an active user session

[Facebook application access token](https://developers.facebook.com/docs/concepts/login/access-tokens-and-types/#appaccess) supports server-to-server communication without an active user session, acting on previously granted permissions for each associated user. Switch to app access tokens for requests not requiring a logged-in Facebook user with an active session tied to the site's app id.

Post to Timeline posts to the Timeline of the post author, not the person who took the post public (only if the post author has authenticated and authorized the Facebook app to post on his or her behalf). Mobile users taking advantage of XML-RPC posting should be able to send their posts to Facebook.

Comments output in a noscript element for SEO purposes no longer load through Facebook PHP methods.

The Facebook PHP SDK is loaded on an as-needed basis and saved to the same `$facebook` global previously referenced for reuse later in the request.

Publish to stream permissions for the current user and Facebook application is no longer verified with each load of a post edit/create screen. The Facebook token for the user is no longer extended with each admin page load.