Fix #504 (#532)

- Add the `spaced` class to some article cards, to make them wider even on desktop
- Fix the CSS to always have a small margin for cards on mobile

static/css/main.scss

@@ -138,4 +138,8 @@ html {
   body > footer * { margin: 1em auto; }
 
   .flex.wrap { flex-direction: column; }
+
+  .cards {
+    margin: 1rem 0 5rem;
+  }
 }