Integrate a router and a session manager

[Meight]

So as to not forget the user or lose track of their position within the application every time they refresh the page.

This would also allow to split Main.elm down into smaller, more manageable and semantically coherent parts. The exact architecture shall be discussed.

This should be done at the client level since we're going for an SPA. elm-spa-example might be a useful source of inspiration.