Refactor hierarchical routing about miappe_api_frontend

[harryle95]

It's not possible (given the current structure) to do this refactoring. The reason is <App/> has an <Outlet/> component that renders children paths' elements inside of it. React router does not seem to allow children element to override parent's element. It merely allows children element to be rendered inside the outlet of parent's element. What this means is if the route hierarchy is

root/investigation/create

There is no way for /investigation/create's element to replace /investigation 's element