[Eventonica] Revise Part 7 - React

[vegetabill]

Resolves #1421

• Fill in Eventonica Part 3 lesson with step-by-step examples of porting a JS DOM app to React
• This should be rebased against Update Eventonica/README to match the current state #1471 first
• Sample code adapted from @lisaau https://github.com/lisaau/Eventonica/blob/master/src/jQuery-scripts.js

[alodahl]

Looks good, I'm just concerned about the class name, and about the re-render comment at the end. Re-rendering on state change is like the main point of React.

[vegetabill]

Thanks for reviewing! Since I have a little cushion of time on this, going to prioritize cleaning up the whole Eventonica mess and will come back to this in a week or so.

[vegetabill]

@alodahl oof I did something dumb with git and now it thinks this is closed but it was never merged to main.

The real changes are now in #1505 including all your suggested revisions.