Subscriptions: submit form via AJAX

### Impacted plugin

Jetpack

### What

Subscribes Block should work with Ajax.

### How

Writing your e-mail address in the Subscribers Block and clicking on the subscribe button refreshes the page, moves the page to the Subscribes Block and displays information. There is no need to refresh the page for this. Let it work with Ajax. It's a relatively simple process anyway. The e-mail is entered, the button is clicked, and information is displayed. Of course I'm not an expert, you know better. But I think it would be very nice to work with ajax.

(sorry, translation...)

Start by locating the Subscribes Block and tracing the subscribe button's form submission and response handling; the issue names no files or tests. Confirm how the current page refresh and status information are implemented, then verify that submitting an email displays the result without refreshing or moving the page.

Written by the indexing model from the issue text.