Remove need of setting more than one facebook_admin

According to Facebook Graph API docs, fb:admins meta tag stands to let
Facebook users manage/view insights[1] and comments[2] of a url. App
administrators are already included in this list, so there is no need to
accept more than one user in the helper.

[1]: https://developers.facebook.com/docs/insights/
[2]: https://developers.facebook.com/docs/plugins/comments/
Rename fb_admins helper to facebook_url_admin