Tidy up admin page routing

[domdomegg]

Currently, I think the routing around the admin pages is quite hacky. It's reach router sitting on top of Gatsby, which doesn't play that nice and adds complexity.

I do think we'd like to keep parameters in the path if possible, so we can share links between people. However, maybe we want to do this a different way - possibly moving this into query parameter routing or similar. Or, figure out how we can do this properly in Gatsby (or Next.js if we want to migrate) in a way that's compatible with our static hosting setup.

Possibly related links:

• https://www.gatsbyjs.com/docs/how-to/routing/client-only-routes-and-user-authentication/
• https://stackoverflow.com/questions/55756994/how-to-create-dynamic-route-in-gatsby
• https://docs.aws.amazon.com/AmazonS3/latest/userguide/how-to-page-redirect.html#advanced-conditional-redirects
• https://stackoverflow.com/questions/55339636/next-js-dynamic-page-params-for-static-export
• https://stackoverflow.com/questions/69004069/how-to-have-a-fallback-like-behavior-for-static-export-in-next-js