Generate all known chords and display randomly selected chords

First of all, Elm's purity is beautiful. I think every language should model
their error messages and develop experience after Elm. If I didn't have to
download packages, I don't think I would need an internet connection to
troubleshoot my program's errors. This is how helpful I find the compiler.

Now that that's out of the way, here's what I've changed since we've last
corresponded:
- Use Elm's Browser.element to create a reactive application with state
- Write a function to generate all of the chords about which CDS knows
- Move some code out of Main.elm into other modules
- Depend on List.Extra, Random, Random.Extra

What's left:
- Lots of work
- Instead of clicking a button to show a new chord, use a timer
- Add mobile-first styling (probably add TailwindCSS)
- Persist settings in LocalStorage (and then eventually create user accounts)
- Allow users to curate the list of chords they're interested in practicing
- Deploy the website and dogfood it

Unknowns:
- How can I handle tempo? I don't expect setInterval to be enough (maybe it
  is)...

website/sandbox/chord-drill-sergeant/elm.json

@@ -8,13 +8,17 @@
         "direct": {
             "elm/browser": "1.0.2",
             "elm/core": "1.0.5",
-            "elm/html": "1.0.0"
+            "elm/html": "1.0.0",
+            "elm/random": "1.0.0",
+            "elm-community/list-extra": "8.2.3",
+            "elm-community/random-extra": "3.1.0"
         },
         "indirect": {
             "elm/json": "1.1.3",
             "elm/time": "1.0.0",
             "elm/url": "1.0.0",
-            "elm/virtual-dom": "1.0.2"
+            "elm/virtual-dom": "1.0.2",
+            "owanturist/elm-union-find": "1.0.0"
         }
     },
     "test-dependencies": {