Following some custom domains leads to subscribing to the underlying wp.com subdomain

I confirmed that I could follow and unfollow WordPress.com Simple sites with custom domains and Jetpack sites as expected.

However, following a WordPress.com Atomic site (e.g. `rachelmcr-test-domain.blog`) or a domain linked to a WordPress.com site through a domain mapping or site redirect record that isn't actually a WordPress site (e.g. `google.com`) leads to unexpected results:

1. Enter the command for an Atomic site, such as `follow https://rachelmcr-test-domain.blog`.
2. The bot responds with the message `Now following rachelmcrtestdomain.wordpress.com` (the underlying subdomain).
3. Publish a post on the site.
4. The bot shares the post in the channel with the correct custom domain.
5. Try to unfollow the site with `unfollow https://rachelmcr-test-domain.blog`.
6. The bot responds with the error `It seems you were not following https://rachelmcr-test-domain.blog`.
7. Try to unfollow the site with the subdomain: `unfollow https://rachelmcrtestdomain.wordpress.com`.
8. The bot responds with the message `No longer following https://rachelmcrtestdomain.wordpress.com`.

This is confusing but also makes it hard to unfollow a blog like this if any time has passed since following it, because the underlying subdomain isn't discoverable.

Start with the follow and unfollow command handlers and the custom-domain resolution path, then reproduce the Atomic-site and non-WordPress-domain cases from the issue. Done means following and unfollowing the custom domain use the same discoverable identity while published posts still appear with the custom domain.

Written by the indexing model from the issue text.