Prevents post type templates ignoring post types due to invalid characters. Each entry in the `Template Post Type` comment is run through `sanitize_key()` to match the sanitisation used by `register_post_type()`.
Fixes#38766.
git-svn-id: https://develop.svn.wordpress.org/trunk@39236 602fd350-edb4-49c9-b593-d223f7449a82
