fix: external link icon relative sizing

Use font size (em) relative sizing for the external link icon, so it
is sized appropriately at the various breakpoints where the nav font
size changes.

Author: jawinn

blocks/header/header.css

@@ -147,7 +147,8 @@ a.nav__home-link {
   }
 
   a.nav-page-links__link--external {
-    --icon-size: 0.625rem;
+    /* Correspond with font size, with ratio of 10px at 18px font size. */
+    --icon-size: 0.625em;
 
     display: flex;
     flex-direction: row;
@@ -160,7 +161,7 @@ a.nav__home-link {
       width: var(--icon-size);
       height: var(--icon-size);
       background: var(--icon-external-link-alt) center no-repeat;
-      background-size: cover;
+      background-size: contain;
       transition: transform ease-out 130ms;
     }