Canonical: Redirect paged requests for a static page assigned as the "Posts page".

This avoids displaying duplicate content of the home page under different URLs with appended page numbers.

This change only affects the `<!--nextpage-->` pagination (`page` query variable) and not the regular multiple posts pagination (`paged` query variable).

The posts page does not support the `<!--nextpage-->` pagination, so requests for invalid page numbers should be redirected to the page permalink, applying the logic previously implemented for single posts or pages.

Follow-up to [34492], [47727].

Props jeremyfelt, sachit.tandukar, SergeyBiryukov.
Fixes #45337. See #40773, #28081, #11694.
Built from https://develop.svn.wordpress.org/trunk@47760


git-svn-id: https://core.svn.wordpress.org/trunk@47536 1a063a9b-81f0-0310-95a4-ce76da25c4cd