Improve nav unification experience on Jetpack sites

[mmtr]

Part of #50225

Let's enhance the nav unification experience on Jetpack sites by implementing the improvements below:

• Update Jetpack_Admin_Menu so we don’t return more items that the ones currently present in the existing navigation.
• Make sure all links point to Calypso (regardless of the link destination toggle).
• Ensure Import, Export, Feedback, and WP Admin links open in new tab and present an "external" icon. These are exceptions to the rule because they will be linking to WP Admin pages because there is no Calypso equivalent.