Routing: Welcome pages do (some) routing manually

[Danziger]

Welcome pages still do routing manually (with conditionals). Instances of this are marked with the following comment: // TODO: Use a nested router instead:

Additionally, navigate() (from useLocation()) can be called with 2 special values on pages where their URL ends in "/" + number, navigate("prev") and navigate("next"), instead of manually calculating the next and previous pages.

This change should also allow the removal all usages of wouter's useRoute() in Welcome views. The params should instead come from the view props (props.params). That requires some views to be refactored to use the router <Routes>.