Add a new conditional tag for the "Posts Page"

[roytanck]

Currently, there's no conditional tag to determine whether the current page is the "Posts Page", set under Settings->Reading. The codex (​https://codex.wordpress.org/Conditional_Tags#The_Blog_Page) describes a way to combine is_home() and is_front_page(), but I propose that we instead add a new conditional tag to deal with this.

Naming the function is a little tricky. Unfortunately, the name "is_posts_page" is taken in query.php, so I created a patch that adds "is_blog_page". It returns true on the "posts page", and false everywhere else.

I've run across this issue in real world projects a couple of times and though it would be a good addition to WordPress. The patch adds this to query.php because is_home() and is_front_page also live there.

[roytanck]

Patch that adds the is_blog_page conditional tag to query.php

[roytanck]

Plugin to test the is_blog_page conditional. Adds a message to the_content that displays whether the current page is the posts page.