Update Material UI to v5

[huijing]

Closes #320 (putting this here to link to issue)

This PR updates Material UI to v5 and makes the necessary syntax changes to support this upgrade. Also removed some redundant code and feeds into https://github.com/WICG/webmonetization/issues/320 Comments have been made on the files tab to further explain specific changes.

🎩 Top-hatting instructions (i.e. human testing that things are not broken)

1. Pull down this branch
2. Run yarn install because dependencies were updated
3. Check the following pages are rendering without error:
   • http://localhost:3000/
   • http://localhost:3000/meta-tag
   • http://localhost:3000/prob-revshare
   • http://localhost:3000/exclusive-content

[sublimator]

Sometimes good to just merge main, then squash when merging Easier to review than force pushes (especially when a PR drags on)