[memberships] Remove token from url

[millerf]

Fixes https://github.com/Automattic/gold/issues/169

Proposed changes:

• Remove adding the token to the URL for no reason
• Do not rely on subscribe=success to know the subscription status of the current user
• Small refactor

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:

WPCOM

• Push this branch on your sandbox and sandbox bestmemberships.wordpress.com
• Open https://bestmemberships.wordpress.com/2023/08/31/access-only-for-free-subscribers/
• From incognito, subscribe for free with a new user
• Do not confirm the email
• Make sure the token is not in the URL
• Make sure you see this:
  
• Reload the page, you should see the same thing, as the cookie should be
• Close the window
• Open a new incognito and re-enter the email, you should see the same
• Answer the confirmation email, you now should have access

Self-hosted

• Create a new JN instance
• Connect Earn to Stripe
• Create a post with Newsletter access and one with a paid-content block
• Pay for the access and when clicking "Continue" make sure the URL does not contain the token

[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 add/memberships-remove-token-from-url branch.

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

  bin/jetpack-downloader test jetpack add/memberships-remove-token-from-url
  

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

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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

• Next scheduled release: November 7, 2023.
• Scheduled code freeze: October 30, 2023.

[chrismccluskey]

I'm not able to see the screenshot you've uploaded to this PR, so I'm guessing a bit as to what the flow is. For some reason, opening a new private window in firefox seems to retain my cookie information, so I augmented the test a bit, changing the last two steps:

• Open a new incognito and re-enter the email, you should see the same
• Answer the confirmation email, you now should have access

To:

• Open a new private window in a separate browser and re-enter the email. (Note: at this point, I got a "Verify your email to continue reading" after entering my login code after entering my email).
• Answer the confirmation email, return to the original post and refresh the page, you should now have access. (Note: at this point, after opening the confirmation link I saw the content; however, when I returned to the original tab where it asked me to verify my email and refreshed that page, I was still greeted with a "Verify your email to continue reading". When I entered my email again on this page, it had me login again, which would repeat the process).

I'm not sure if this behavior is expected or if this just needs to be addressed in another patch. As far as being able to see the content, I am able to do so, because the confirmation page shows the content.

[pottedmeat]

Code looks good and tested successfully.