Admin: check that site is publicly accesible by WPCOM before activating certaing modules

Follow up to https://github.com/Automattic/jetpack/pull/5609

1. the code removed in that PR should be restored so the checking is performed, but,
2. we should allow users to activate these modules and display a message telling user that it might not work as intended since the site is not publicly accesible by WordPress.com

The Sitemaps module does something similar (although it doesn't check that is publicly accessible by WPCOM) when the site is set to discourage search engines: it can be activated but it displays a message saying it will not work as intended until the site is accesible by search engines.

cc @zinigor @kraftbj