#7669271 Partner Portal: empty state design for the assign licenses page

Proposed Changes

This PR updates the empty state design for the assign licenses page.

Testing Instructions

Prerequisites

You should have no sites available. You can signup as a new user.

NOTE: Make sure you add a payment method by visiting http://jetpack.cloud.localhost:3000/partner-portal/payment-methods.

1. Run git checkout update/empty-site-state-for-assign-licenses-page and yarn start-jetpack-cloud
2. Open http://jetpack.cloud.localhost:3000/, and you’ll be redirected to the /dashboard.
3. Click on Licensing on the top nav -> Click on Licenses on the left nav -> Go to the Unassigned tab -> Click on Assign License on any license item -> Verify that the new empty site design is shown as below and the buttons work as expected.

Before	After

4. Verify it looks well on mobile and tab view

Mobile view

Tab view

5. Verify the site selector as shown below when there are sites(You might have to refresh the page).

Pre-merge Checklist

• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the “[Status] Needs Privacy Updates” label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?

Related to 1202619025189113-as-1201942917845813