Render a mobile-friendly piano

For now since I'm the only customer and I'm primarily making this for myself,
I'm styling the app specifically for my Google Pixel 4. If I find this app
useful, I will consider supporting other devices.

I'm using the Icons that I bought when I purchased the "Refactoring UI" book.

Other news:
- I bought the domain learnpianochords.app!

What's left:
- Style the "fine tune" tab of the preferences view
- Better support non-mobile devices like the browser and tablet devices
- Deploy the application to learnpianochords.app
- Redesign the "key" tab of the preferences view to sort the keys according to
  the circle of fifths
- Dogfood
- Simplify until I cannot simplify anymore

website/sandbox/chord-drill-sergeant/src/UI.elm

@@ -79,7 +79,7 @@ textToggleButton { label, toggled, handleClick, classes } =
         buttonClasses =
             [ textColor
             , textTreatment
-            , "py-10"
+            , "py-8"
             , "px-10"
             , "text-5xl"
             ]