Port basket newsletter JS to Protocol (Fixes #839)

[alexgibson]

Description

This PR ports the JS code (and tests) from bedrock that handles newsletter form submissions.

This will help provide Mozilla websites with an up to date example of how to handle newsletter submissions that post directly to Basket, and will act as a replacement for https://github.com/mozilla/basket-example.

I also fixed a couple of other pesky long time bugs, like how small <legend> elements were rendered in Zilla Slab, making them hard to read.

• I have documented this change in the design system.
• I have recorded this change in CHANGELOG.md.

Issue

#839

Testing

• http://localhost:3000/components/detail/newsletter--default
• http://localhost:3000/components/detail/newsletter--errors
• http://localhost:3000/components/detail/newsletter--success

[reemhamz]

It works!
I tested it out, and I saw the form submission go to Basket in dev tools.

However, instead of a POST request, I'm getting an OPTIONS request. Is this expected? I'm assuming that's the case because it's determining if the URL it wants to POST to is safe to send?

In any case, it just needs a rebase. r+(wc?) 🍅

[alexgibson]

@reemhamz thanks for the review! (rebased)

The way CORS (cross origin) requests work is that browsers will first send an OPTIONS pre-flight request to make sure the site is allowed to make the cross origin request. If allowed, it will then make the POST. You should hopefully see two requests in the network console?

[reemhamz]

Ah yup, I see both requests in the console :) so this is good to merge, but I'll leave that to you! 👍🏼

[alexgibson]

Thanks for double checking!