UX: restore table borders

Newer versions of Normalize remove the `border-collapse: collapse;` property and fallback to browser defaults. This commit restores that property because we're using it in quite a few places.

app/assets/stylesheets/common/foundation/base.scss

@@ -124,6 +124,10 @@ pre code {
 // Tables
 // --------------------------------------------------
 
+table {
+  border-collapse: collapse;
+}
+
 tbody {
   border-top: 3px solid $primary-low;
 }