Subscribe block: return to correct page after subscribing even when not in a post

[simison]

Follow up to https://github.com/Automattic/jetpack/pull/33932/files#r1384990526

Improves redirection to correct place which happens after confirming subscription.

Previously we easily sent users to subscription management, which isn't great if they started from the site:

Proposed changes:

• Adds individual page IDs to subscription modal, and defaults to "page on front" id (static homepage or "blog" page) if none is found.

The ID will be used to pull paid status of the post object (which of course is available for blog posts only), but also to pull "redirect URL" which we send reader to after they confirm subscription. Otherwise they end up in Calypso subscription management.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Create free and paid posts, test subscribing to those. After confirming subscription, you should land at the post.
• Subscribe from subscription block outside the posts loop, e.g. on homepage header/footer (Use Lettre theme for example) — land at homepage after confirming subscription.
• Subscribe from a page — land at the page after confirming subscription.

[github-actions]

Thank you for your PR!

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• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
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Jetpack plugin:

The Jetpack plugin has different release cadences depending on the platform:

• WordPress.com Simple releases happen daily.
• WoA releases happen weekly.
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[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the update/subscriptions-post-id branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack update/subscriptions-post-id
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[TimBroddin]

The code looks really good! Tested & approved.

[lezama]

nice and simple