See 69-gh-wp-calypso

The REST API settings endpoint includes a property jetpack_relatedposts_enabled, which is true if the Related Posts plugin will display related posts. The value from this endpoint is incorrect, however, when a site has not modified their settings. This is because the Jetpack_RelatedPosts plugin defaults the setting to be enabled if it is not set.

This change modifies the endpoint to use the same logic: if the setting is not set, it will default to reporting jetpack_relatedposts_enabled as true.

Some sites (VIP and Jetpack) default this setting to false instead, but the way this is accomplished is that when the plugin is loaded by wp-content/mu-plugins/related-posts.php, it actively changes the option to false. Since that loader should still be used by the REST API, that should mean that such sites will continue to have jetpack_relatedposts_enabled reported correctly.

This commit syncs r159322-wpcom.

Testing instructions:

1. Choose a site which has not changed its Related Posts setting.
2. Visit this wp-admin page on that site: https://[your-site-here]/wp-admin/options-reading.php
3. Verify that the Related posts setting reads "Show related content after posts".
4. BEFORE applying this patch...
5. Using a REST API client (eg: the WPCOM developer console), query the following endpoint: GET /rest/v1.1/sites/[your-site-here]/settings
6. Verify that settings > jetpack_relatedposts_enabled is false.
7. Apply this patch and make sure to sandbox the REST API.
8. Using a REST API client (eg: the WPCOM developer console), query the following endpoint: GET /rest/v1.1/sites/[your-site-here]/settings
9. Verify that settings > jetpack_relatedposts_enabled is true.
10. On the wp-admin page, change the setting to read "Hide related content after posts" and click "Save Changes" at the bottom.
11. Using a REST API client (eg: the WPCOM developer console), query the following endpoint: GET /rest/v1.1/sites/[your-site-here]/settings
12. Verify that settings > jetpack_relatedposts_enabled is false.