#7787529 Change behaviour of dotorg theme nudge

Proposed Changes

Link the dotorg theme nudge displayed to sites that are not atomic
capable straight to the checkout for the business plan.

Include a redirect so that post-purchase, the user is returned back to
the themes page so they can then activate the theme.

Make the nudge wording clearer so that what you are buying is more
explicit.

Testing Instructions

1. Use live link go to /themes on a < business site
2. Search for a dot org only theme (not a TTn theme)
3. Click on the theme
4. You should see the theme detail page, including a nudge saying ‘Upgrade your plan to access third-party themes’ and directly mentioning the business plan.
5. Click the nudge and you should get sent to the checkout screen for a business plan
6. After checkout you should get redirected back to the theme with a notification in the top right saying words to the effect of ‘purchase complete’. The nudge shouldn’t appear and instead an ‘Activate’ button should.

Repeat from the logged out theme showcase, this should go via the plans page instead so you can choose / create a site after you register or login.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• 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 Automattic/dotcom-forge#1299