Subscribe block: handle confirmation for two blocks at the same page

Via p9Jlb4-787-p2

@crisbusquets wrote:

> When subscribing, the behaviour is a bit strange: it shows a success alert, the input and, at the same time, it automatically scrolls to the posts.
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> This can create a strange interface if the newsletter doesn’t have many posts yet — in this case, there are two alerts in the same viewport.

Start by reproducing the behavior with two Subscribe blocks on the same page, including a newsletter with few posts, and observe the success alerts, input, and automatic scroll. Done means the confirmation experience no longer leaves two alerts in the same viewport; the issue names no files or tests, so the relevant Subscribe block entry point must first be located.

Written by the indexing model from the issue text.