Add styles for highlight code blocks

Set it so that code blocks that have been highlighted are bound to their
<pre> containers.

static/css/base.css

@@ -76,6 +76,9 @@
     code {
         @apply inline-block px-1 bg-gray-50 rounded-sm;
     }
+    .highlight > pre > code {
+        @apply w-full overflow-auto;
+    }
 
     hr {
         @apply w-full border-gray-700 mt-4 mb-2;