Replace router.js with Turbo

[gravitystorm]

We have a router.js file, which allows page navigation without full page reloads. However, this functionality is also provided, along with a lot more things, by Turbo.

We should figure out how to remove our router.js. This would ideally be done incrementally, but if necessary could be implemented in one larger change. I don't yet have enough experience with Turbo to suggest the best approach.