components+hooks: refactor window measurement

We relocated window measurment logic into a hook, useGridSpans(), which
tells us the appropriate number of columns for the IconGrid. This is
useful for rendering an InfoPanel in the right place! In future we
should look into a custom implementation using ResizeObserver and
polyfills, and that may be extensible for other use-cases.

src/hooks/useGridSpans.ts

@@ -0,0 +1,13 @@
+import { useWindowSize } from "react-use";
+
+const GRID_PADDING = 32; // .grid-container { padding }
+const TOOLBAR_WIDTH = 17; // IS THIS BROWSER-SPECIFIC?
+const MAX_GRID_WIDTH = 1120; // .grid { max-width }
+const ITEM_WIDTH = 168; // .grid-item { width; height; margin }
+
+export default (): number => {
+  const { width } = useWindowSize();
+  return Math.floor(
+    Math.min(width - GRID_PADDING - TOOLBAR_WIDTH, MAX_GRID_WIDTH) / ITEM_WIDTH
+  );
+};