Fix height of dashboard counters (#11736)

app/javascript/styles/mastodon/dashboard.scss

@@ -15,6 +15,8 @@
       padding: 20px;
       background: lighten($ui-base-color, 4%);
       border-radius: 4px;
+      box-sizing: border-box;
+      height: 100%;
     }
 
     & > a {