Begin styling efforts

Start styling the Chord Drill Sergeant for mobile devices because that is that
device on which I will primarily use CDS.

I'm also deleting the debugger related code. I would like to support a debugger,
but I'm not currently using this one, so I am going to remove it to keep things
slender.

- Introduce TailwindCSS, which also introduced elm-live, index.html, index.css
- Add mobile-first styling for the preferences modal
- Remove unused code

website/sandbox/chord-drill-sergeant/shell.nix

@@ -4,5 +4,6 @@ in pkgs.mkShell {
   buildInputs = with pkgs; [
     elmPackages.elm
     elmPackages.elm-format
+    elmPackages.elm-live
   ];
 }