Subscribe block: improve confirmation

Via p9Jlb4-787-p2

@crisbusquets wrote:

> The alerts are not distinct enough, as they appear as a regular paragraph. I think it would be better to implement something like this
>
> 

Jeremy shared:

> A visual checkmark would be good, with some sort of smaller and simpler text message (maybe in a colored box?) like “You’re all set! Check your inbox to confirm your subcription.” And without the page refresh would be great.

See further design convo p9Jlb4-787-p2#comment-7736 (Todo: copy it to here without internal references)

We could use [Interactivity API](https://make.wordpress.org/core/2023/03/30/proposal-the-interactivity-api-a-better-developer-experience-in-building-interactive-blocks/) to make the block more dynamic.

Start by locating the Subscribe block and read the linked Interactivity API proposal and referenced design discussion, since no source files or tests are named here. Confirm the desired checkmark, shorter confirmation text, colored treatment, and no-refresh behavior with the issue’s design context; done means the confirmation is visually distinct and appears without a page refresh.

Written by the indexing model from the issue text.