Retrieve parent_title using REST API

[IreneStr]

Currently, we're using jQuery to retrieve the parent title of pages and categories (see https://github.com/Yoast/wordpress-seo/blob/trunk/js/src/wp-seo-replacevar-plugin.js#L494). We should use the REST API instead to make it work in Gutenberg as well.

Setting a parent

Pages

• In the classic editor, you can set a parent for a page in the metabox on the right-hand side.
• In Gutenberg, you can set a parent for a page in the Document sidebar under the 'Page Attributes' tab.

Categories

• In the classic editor, you can set a parent for a category under 'Edit Category'

WordPress REST API

WordPress REST API handbook: https://developer.wordpress.org/rest-api/reference/