Submissions should be blocked while a request is in process

[pushred]

Currently it's possible to send multiple requests to a service by submitting the subscription form once or more while a previous request is still underway. This is usually handled on the backend with validations and brute force precautions, but can sometimes this can impact the user such as with MailChimp, which enforces a rate limit.

Slow internet connections, intermittent backend delays and errors, user confusion (double-clicking), and other reasons can lead to multiple submissions. Providing indeterminate progress feedback could help, but to be safe, subscribe-email should block submissions while a request is underway to avoid any unexpected behavior.