Move Jetpack::disconnect_site() into connection package

[gravityrail]

Right now, there's an empty disconnect_site method in Automattic\Jetpack\Connection\Manager. All the logic lives in the static method Jetpack::disconnect_site() in class.jetpack.php.

This requires that consumers of the connection package implement their own disconnect logic.

We have an issue here, which is that if you have two or more plugins installed that use the connection, disconnecting one may disconnect the others. We should come up with a solution here, preferably one that gives the user clear choices:

1. "This connection is shared with these these other plugins. Would you prefer to deactivate this plugin instead?"

or maybe we only display the disconnect button if there aren't other connection package consumers installed? Or something else?