#5488792 Partner Portal: Add masterbar navigation to switch between managing sites and the portal

Context:

• Project thread: pbtFPC-Um-p2
• i4 designs: pbtFPC-103-p2

Changes proposed in this Pull Request

• Add a navigation to the masterbar in Calypso Green to switch between managing sites and the portal.

Testing instructions

• Checkout PR locally and run yarn && yarn start-jetpack-cloud.
• Open up the local env in your browser (the url should be listed in the yarn output when it’s ready).
• If you have a partner key (please contact Infinity if you wish to test this) makes sure the navigation DOES appear:
  
• If you don’t have any partner keys, make sure the navigation DOES NOT appear.
  • If you already have keys you can fake the response by editing getJetpackPartnerPortalPartners in client/lib/wpcom-undocumented/lib/undocumented.js to return early: return Promise.resolve([]);
• Make sure toggling between “Manage Sites” and “Partner Portal” toggles the current page accordingly.
• Make sure the correct page is highlighted as active when navigating, refreshing etc.