[Breaking] Allow full routes for RouteToUrl in obelisk widgets

[alexfmpe]

There's currently no type-safe way to obtain a url for a backend route on frontend widgets. Current workaround is to do Right validEncoder = checkEncoder fullRouteEncoder on the frontend and use renderBackendRoute.

This PR makes ObeliskWidget provide RouteToUrl for the full route, but breaks backwards compatibility. If only frontend routes are used, apps can simply provide an adapter at the root

  _frontend_body = do
    ...
    fullRouteToUrl <- askRouteToUrl
    let frontendRouteToUrl = fullRouteToUrl . hoistR (FullRoute_Frontend . ObeliskRoute_App)
    runRouteToUrlT myApp frontendRouteToUrl

but to actually use this for backend routes, one needs to propagate RouteToUrl constraints throughout the app, and adjust the arguments of all the routeLink family of widgets. Maybe some local equivalent for RouteToUrl would make the transition smoother by allowing an easy way to gradually rollout the expanded functionality.

I have:

• [x] Based work on latest develop branch
• [ ] Followed the contribution guide
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• [ ] Run the test suite: $(nix-build -A selftest --no-out-link)
• [ ] Updated the changelog
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