Use common Layout for NextJS

[ThePhisch]

NextJS has support for common layouts, as opposed to our current setup of having a layout.tsx in each folder of src/app/{pagename}, in which the layout page is essentially unchanged each time.

Besides having a better folder structure and fewer redundant files (we could even rework the entire routing system this way), the benefit is that the Layout component can then keep its state, even when switching routes.

Helpful material: https://nextjs.org/docs/pages/building-your-application/routing/pages-and-layouts