Remove React Router

[JohnForster]

React Router is industry standard, and very powerful, but in the past has been a big stumbling block to understanding what's going on in the template repo. This PR removes it, instead opting for a single state value that determines which page to render.

Hopefully this will help reinforce the idea of the frontend as a Single Page Application, distinguishing it from the types of applications learners have built previously. Adding (and understanding!) React Router could then be a ticket on the Acebook Trello.

I'm not 100% sure about this change, because it does make the application harder to use. Specifically, not being able to use the back button is a pain, but maybe this pain is actually a good thing, as they will understand the utility of React Router once they add it?

Definitely one for discussion, so please let me know your thoughts.

[ClaireMakers]

Would you say the last group you ran Acebook with really struggled with the React Router and the notion that these aren't real "routes" to other pages, but more like a switchboard of which components to display?

I don't know if the groups would manage to add it from scratch themselves, converting this version with a hook into the router itself, or if they'd see the point in doing it that way? While it might feel difficult when it's already implemented, having the code there and figuring out how to use it is also one way of learning about React Router? I'm not 100% sure either what approach would be better :/