Introduces `is_post_status_viewable()` as a sibling to `is_post_type_viewable()`. Internal and protected statuses are never considered viewable. For built in posts statuses the `public` attribute is checked, for custom statuses the `publicly_queryable` attribute is checked.
Introduces `is_post_publicly_viewable()` for determining if an individual post can be viewed by logged out users. A post is considered viewable if both `is_post_status_viewable()` and `is_post_type_viewable()` return `true` for the post's attributes.
Additionally modifies `is_post_type_viewable()` to return `false` if an unregistered post type is passed to the function to avoid attempting to access properties on a non-object.
Props peterwilsoncc, SergeyBiryukov, whyisjake, TimothyBlynJacobs.
Fixes#49380.
git-svn-id: https://develop.svn.wordpress.org/trunk@50130 602fd350-edb4-49c9-b593-d223f7449a82
