Notice: rename Warn -> Notice and add composability

The Notice component can now act as a general indicator component,
exposing four types: "wait", "help", "warn", or "none". These types
display different icons and make this a much more reusable element.

We also render children passed to us, making this even more extendable.
In future, we may wish to extract all message props altogether as
children instead.

src/components/App/App.css

@@ -25,7 +25,6 @@ pre,
 code {
   font-family: "IBM Plex Mono", "Courier New", monospace;
   font-size: 14px;
-  color: "black";
 }
 
 pre {