Updates react-scripts to v4, eslint to v7

[mattxwang]

This PR does a couple of things:

• update react-scripts (the package for create-react-app) to v4.0.3 - this is a major breaking change, but also helps with Bump node-sass from 4.14.1 to 5.0.0 #130
• update eslint to v7+, to resolve newly-created peer-dependency mismatches
• removes some unused CSS I forgot to remove from Swapping icons to Font Awesome, improves icon accessibility #148
• changes the about diamond to a src/assets-imported image rather than using a background-image hack (the nice externality is that it's also more accessible)!

Closes #136.

[mattxwang]

As a note, we're failing a CI error from an issue with sass not resolving images in the public loader. This is an open issue on the CRA repo, and I don't want to bundle the image into our stylesheet if possible.

facebook/create-react-app#10022

facebook/create-react-app#9937

Curious if other peeps have thoughts on this; there's no burning reason to switch over yet, though we'll have to deal with it soon.

FWIW, this is the problem line:

website/src/components/About/style.scss

Line 43 in fd3095a

background-image: url(/images/about1.png);

[evanzhong]

on the most part lgtm.

Things I note:

• Merge conflict on package-lock.json is pretty gnarly, can just force push if you want as you listed in PR description.
• Personally I'm not opposed to just bundling the image if it's just this one reference to background image.
• Obviously failing integration tests not ideal but from discussions you linked fix does not seem that bad (moving from public to src folders)

[mattxwang]

Update - turns out our icons also use this pathing 😢 . I'll update them first, then circle back.

[mattxwang]

Hm, I am ... messing up this force-merge. Sorry for the notifs Evan, will re-push.