Improve Newsletter Form Submission: Add Error Icon and Use Fetch API with Async/Await

[Precious-Bob]

Good day Kamil, I tried subscribing to the NestJS newsletter and noticed that the success icon activates even when the POST request fails. To improve the user experience and code quality, I would love to work on the following changes:

Add an Error Icon: Introduce an error icon to inform users if their subscription request fails, providing clear feedback on the submission status.
Use Fetch API with Async/Await: Update the codebase to utilize the Fetch API and async/await for handling the form submission, replacing the existing method for better readability and modern JavaScript practices.

[kamilmysliwiec]

PRs are more than welcome!

[apoorvpathak]

a spinner would be nice too, i am working on the PR

[kamilmysliwiec]

Let's track this here #124