Update Facebook login components to a hybrid JS and PHP model

This diff updates the Facebook account management features in the edit profile screen, introduces client-side page selectors, and saves a long-lived access token for a selected page.

Associating a WordPress user account with a Facebook account still happens in the user profile page. A WordPress user can choose to [associate a Facebook account with no additional permissions](https://developers.facebook.com/docs/reference/login/public-profile-and-friend-list/) to add Facebook-specific authorship data and [Facebook Comments Box](https://developers.facebook.com/docs/reference/plugins/comments/) comments moderation capability to posts authored by that user. A WordPress user may also choose to grant the WordPress site's Facebook application permission to post to his or her Timeline on his or her behalf including a personal options checkbox opting-in to the post to author timeline post meta box in the post editor.

Associating a Facebook Page with a site still happens in the social publisher settings page. A site with an associated Facebook Page will continue to see the page name on the settings page. A WordPress with a connected Facebook account will see either a prompt to grant [Facebook page permissions](https://developers.facebook.com/docs/reference/login/page-permissions/) to the WordPress site's associated Facebook app or see a list of Facebook pages which have granted the current Facebook user permission to create content. Newly associated pages will have a freshly requested, long-lived access token associated saved for use by the site and its Facebook app.

The old `admin/login.php` called on edit post initializations is now gone. Edits occur on the profile page for Facebook account information and the social publisher settings page. If your code extended or relied on previous login JS or PHP it will most likely need updating.