Related Posts: provide per post type check box to enable/disable

Currently, Related Posts will show on any post and there are a range of [customization options](https://jetpack.com/support/related-posts/customize-related-posts/#woocommerce).

One of these could be turned into a feature identical to Likes and Sharing:

Image link: https://cloudup.com/curVvgbXn8W

Proposal - add UI section to Settings or Customizer to allow per post type to control if Related Posts should be shown.

Customizer:
Image link: https://cloudup.com/co5NjuDZC17

Reading Settings:
Image link: https://cloudup.com/csopaQT9qVT

Primarily this would be to add support for WooCommerce Products since there is the feature [Related Products](https://docs.woocommerce.com/document/related-products-up-sells-and-cross-sells/#section-3) and adds support for [Related posts on Pages](https://jetpack.com/support/related-posts/customize-related-posts/#display-on-pages).

Forum thread which started this - https://wordpress.org/support/topic/related-posts-plugin-shows-in-product-pages/

Start by reviewing the existing Related Posts customization options and the comparable Likes and Sharing controls, then trace how Related Posts decides which post types to display. Done means a Settings or Customizer control can enable or disable Related Posts per post type, including Pages and WooCommerce Products, with the selected behavior reflected on the site.

Written by the indexing model from the issue text.