Some Jetpack settings/endpoints not available in Calypso when using standalone plugins

## Summary

As an example, when using a standalone plugin like Jetpack Social on a site connected to WordPress.com, the Sharing Buttons (which require Jetpack proper) can't be enabled from Calypso and give a non-helpful error:

An additional example I noticed was trying to modify the the SEO settings (also requires Jetpack plugin) on `https://wordpress.com/marketing/traffic/example.com`

**We should audit other Calypso use cases here and work on improving messaging or updating the logic on those pages to account for sites with a full Jetpack connection, but not using the Jetpack plugin.**

Source: 7832-gh-Automattic/jpop-issues

Start by reproducing the Sharing Buttons flow in Calypso with Jetpack Social installed, then check the SEO settings at wordpress.com/marketing/traffic/example.com. Audit the other Calypso use cases affected by a full Jetpack connection without the Jetpack plugin. Done means those pages either account for the connection state or show helpful messaging instead of the current error.

Written by the indexing model from the issue text.