Subscriptions / AMP standard: success page is unreachable on HTTP sites

#### Steps to reproduce the issue

Start from a site:
- Connected to WordPress.com
- With Subscriptions module active
- With the AMP plugin active
- With AMP's standard mode active
- Using HTTP (not HTTPS)

1. Go to Posts > Add New
2. Publish a new post with a Subscriptions block
3. In an incognito, subscribe to your site with that block.
4. Upon successfully submitting the form, the page will reload with the `?subscribe-success` parameter, but this will be a 404 page.

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p8oabR-za-p2#comment-4733

Start by tracing the Subscriptions block submission flow and handling of the `?subscribe-success` parameter when AMP standard mode is active on an HTTP site. Reproduce the steps in the issue and verify that a successful subscription reaches the success page instead of a 404.

Written by the indexing model from the issue text.