Subscription Block: update subscriber count to take Publicize settings into account

The Subscription Block includes Publish panels that show the number of followers that will be impacted by publishing that post:

On WordPress.com Simple sites, if the site uses the Publicize feature, the total count of subscribers includes the number of followers from each connected social network.

While this makes sense, it can be problematic when one has disabled sharing for one or more connected social networks for that post. In this situation, the count should be updated accordingly.

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Originally suggested in #23688

Start by tracing the Subscription Block's Publish panels and how they obtain the subscriber count, then identify where Publicize settings for connected social networks are available. Done means the displayed count excludes networks disabled for the post while retaining enabled networks; verify the behavior for multiple connected networks.

Written by the indexing model from the issue text.